How to merge 3 images togehter stacked in a batch?

You can use a scripting language like Python to achieve this. Here's a step-by-step approach:

Install the required libraries by running the following command in your terminal:

pip install numpy opencv-python

1. Create a new Python script in the same directory where your images are stored. Let's name it image_merge.py.
2. Open image_merge.py in a text editor and add the following code: 

import os
import cv2
import numpy as np

# Set the input and output directories
input_dir = 'input_images/'
output_dir = 'output_images/'

# Create the output directory if it doesn't exist
if not os.path.exists(output_dir):
    os.makedirs(output_dir)

# Get the list of image filenames in the input directory
image_files = sorted([f for f in os.listdir(input_dir) if f.endswith('.jpg')])

# Iterate over the image files and merge them in stacks of three
for i in range(0, len(image_files), 3):
    # Load the three images
    image1 = cv2.imread(os.path.join(input_dir, image_files[i]))
    image2 = cv2.imread(os.path.join(input_dir, image_files[i + 1]))
    image3 = cv2.imread(os.path.join(input_dir, image_files[i + 2]))

    # Stack the images vertically
    merged_image = np.vstack((image1, image2, image3))

    # Save the merged image with a new name
    output_filename = f"merged_{i // 3}.jpg"
    cv2.imwrite(os.path.join(output_dir, output_filename), merged_image)

print("Image merging completed.")

1. Adjust the input_dir and output_dir variables in the script to match the paths of your input and output directories, respectively.
2. Save the script and run it using the following command in your terminal:

python image_merge.py

The script provided automatically merges images in stacks of three, saving the merged images with a "merged_" prefix and a stack index in the output directory. Input images should be named to ensure correct alphanumeric sorting. The script supports JPEG format by default but can be modified for other formats.

This will help you. 

@HeyoThere 

The following script is complete for your use case, no need for an action to be run by the script:

/* 

https://community.adobe.com/t5/photoshop-ecosystem-discussions/how-to-merge-3-images-togehter-stacked-in-a-batch/td-p/13922876
Stack 3 Document Sets to Vertically Stacked Layers.jsx
Stephen Marsh
9th July 2023

This script requires input files from a single folder to be alpha/numeric sorting in order to stack in the correct set quantity. 

*/

#target photoshop

if (!app.documents.length) {

    try {

        // Save and disable dialogs
        var restoreDialogMode = app.displayDialogs;
        app.displayDialogs = DialogModes.NO;

        // Main script function
        (function () {

            // Select the input folder
            var inputFolder = Folder.selectDialog('Please select the folder with files to process');
            if (inputFolder === null) return;

            // Limit the file format input, add or remove as required
            var fileList = inputFolder.getFiles(/\.(png|jpg|jpeg|tif|tiff|psd|psb)$/i);

            // Force alpha-numeric list sort
            // Use .sort.reverse() for the first filename in the merged file
            // Use .sort() for the last filename in the merged file
            fileList.sort();

            var setQty = 3;

            // Validate that the file list is not empty
            var inputCount = fileList.length;
            var cancelScript1 = (inputCount === 0);
            if (cancelScript1 === true) {
                alert('Zero input files found, script cancelled!');
                return;
            }
            // Validate the input count vs. output count - Thanks to Kukurykus for the advice to test using % modulus
            var cancelScript2 = !(inputCount % setQty);
            alert(inputCount + ' input files stacked into sets of ' + setQty + ' will produce ' + inputCount / setQty + ' output files.');
            // Test if false, then terminate the script
            if (cancelScript2 === false) {
                alert('Script cancelled as the quantity of input files are not evenly divisible by the set quantity.');
                return;
            }

            // Select the output folder
            var outputFolder = Folder.selectDialog("Please select the folder to save to");
            if (outputFolder === null) {
                alert('Script cancelled!');
                return;
            }

            // or

            /*
            // Create the output sub-directory
            var outputFolder = Folder(decodeURI(inputFolder + '/Output Sets Folder'));
            if (!outputFolder.exists) outputFolder.create();
            */

            // Set the file processing counter
            var fileCounter = 0;

            // Loop through and open the file sets
            while (fileList.length) {
                // Sets of N quantity files
                for (var a = 0; a < setQty; a++) {
                    try {
                        app.open(fileList.pop());
                    } catch (e) { }
                }

                // Set the base doc layer name
                app.activeDocument = documents[0];
                docNameToLayerName();

                // Stack all open docs to the base doc
                while (app.documents.length > 1) {
                    app.activeDocument = documents[1];
                    docNameToLayerName();
                    app.activeDocument.activeLayer.duplicate(documents[0]);
                    app.activeDocument = documents[0];
                    app.documents[1].close(SaveOptions.DONOTSAVECHANGES);
                }

                ////////////////////////////////// Start doing stuff //////////////////////////////////

                var idcanvasSize = stringIDToTypeID( "canvasSize" );
                var desc295 = new ActionDescriptor();
                var idheight = stringIDToTypeID( "height" );
                var idpercentUnit = stringIDToTypeID( "percentUnit" );
                desc295.putUnitDouble( idheight, idpercentUnit, 300.000000 );
                var idvertical = stringIDToTypeID( "vertical" );
                var idverticalLocation = stringIDToTypeID( "verticalLocation" );
                var idtop = stringIDToTypeID( "top" );
                desc295.putEnumerated( idvertical, idverticalLocation, idtop );
                executeAction(idcanvasSize, desc295, DialogModes.NO);

                activeDocument.selection.selectAll();

                var idselect = stringIDToTypeID( "select" );
                    var desc393 = new ActionDescriptor();
                    var idnull = stringIDToTypeID( "null" );
                        var ref95 = new ActionReference();
                        var idlayer = stringIDToTypeID( "layer" );
                        var idordinal = stringIDToTypeID( "ordinal" );
                        var idbackwardEnum = stringIDToTypeID( "backwardEnum" );
                        ref95.putEnumerated( idlayer, idordinal, idbackwardEnum );
                    desc393.putReference( idnull, ref95 );
                    var idmakeVisible = stringIDToTypeID( "makeVisible" );
                    desc393.putBoolean( idmakeVisible, false );
                    var idlayerID = stringIDToTypeID( "layerID" );
                        var list14 = new ActionList();
                        list14.putInteger( 3 );
                    desc393.putList( idlayerID, list14 );
                executeAction(idselect, desc393, DialogModes.NO);

                var idalign = stringIDToTypeID( "align" );
                    var desc396 = new ActionDescriptor();
                    var idnull = stringIDToTypeID( "null" );
                        var ref96 = new ActionReference();
                        var idlayer = stringIDToTypeID( "layer" );
                        var idordinal = stringIDToTypeID( "ordinal" );
                        var idtargetEnum = stringIDToTypeID( "targetEnum" );
                        ref96.putEnumerated( idlayer, idordinal, idtargetEnum );
                    desc396.putReference( idnull, ref96 );
                    var idusing = stringIDToTypeID( "using" );
                    var idalignDistributeSelector = stringIDToTypeID( "alignDistributeSelector" );
                    var idADSCentersV = stringIDToTypeID( "ADSCentersV" );
                    desc396.putEnumerated( idusing, idalignDistributeSelector, idADSCentersV );
                    var idalignToCanvas = stringIDToTypeID( "alignToCanvas" );
                    desc396.putBoolean( idalignToCanvas, false );
                executeAction(idalign, desc396, DialogModes.NO);

                var idselect = stringIDToTypeID( "select" );
                    var desc393 = new ActionDescriptor();
                    var idnull = stringIDToTypeID( "null" );
                        var ref95 = new ActionReference();
                        var idlayer = stringIDToTypeID( "layer" );
                        var idordinal = stringIDToTypeID( "ordinal" );
                        var idbackwardEnum = stringIDToTypeID( "backwardEnum" );
                        ref95.putEnumerated( idlayer, idordinal, idbackwardEnum );
                    desc393.putReference( idnull, ref95 );
                    var idmakeVisible = stringIDToTypeID( "makeVisible" );
                    desc393.putBoolean( idmakeVisible, false );
                    var idlayerID = stringIDToTypeID( "layerID" );
                        var list14 = new ActionList();
                        list14.putInteger( 3 );
                    desc393.putList( idlayerID, list14 );
                executeAction(idselect, desc393, DialogModes.NO);

                var idalign = stringIDToTypeID( "align" );
                    var desc399 = new ActionDescriptor();
                    var idnull = stringIDToTypeID( "null" );
                        var ref98 = new ActionReference();
                        var idlayer = stringIDToTypeID( "layer" );
                        var idordinal = stringIDToTypeID( "ordinal" );
                        var idtargetEnum = stringIDToTypeID( "targetEnum" );
                        ref98.putEnumerated( idlayer, idordinal, idtargetEnum );
                    desc399.putReference( idnull, ref98 );
                    var idusing = stringIDToTypeID( "using" );
                    var idalignDistributeSelector = stringIDToTypeID( "alignDistributeSelector" );
                    var idADSBottoms = stringIDToTypeID( "ADSBottoms" );
                    desc399.putEnumerated( idusing, idalignDistributeSelector, idADSBottoms );
                    var idalignToCanvas = stringIDToTypeID( "alignToCanvas" );
                    desc399.putBoolean( idalignToCanvas, false );
                executeAction(idalign, desc399, DialogModes.NO);

                activeDocument.selection.deselect();

                ////////////////////////////////// Finish doing stuff //////////////////////////////////

                // Delete XMP metadata to reduce final file size of output files
                removeXMP();

                // Save name + suffix & save path
                var Name = app.activeDocument.name.replace(/\.[^\.]+$/, '');
                var saveFile = File(outputFolder + '/' + Name + '_x' + setQty + '-Sets' + '.psd');
                // var saveFile = File(outputFolder + '/' + Name + '_x' + setQty + '-Sets' + '.jpg');

                // Call the save function
                savePSD(saveFile);
                //saveTIFF(saveFile);
                //saveJPEG(saveFile);
                //savePNG(saveFile);

                // Close all open files without saving
                while (app.documents.length) {
                    app.activeDocument.close(SaveOptions.DONOTSAVECHANGES);
                }

                // Increment the file saving counter
                fileCounter++;


                ///// Functions /////

                function savePSD(saveFile) {
                    psdSaveOptions = new PhotoshopSaveOptions();
                    psdSaveOptions.embedColorProfile = true;
                    psdSaveOptions.alphaChannels = true;
                    psdSaveOptions.layers = true;
                    psdSaveOptions.annotations = true;
                    psdSaveOptions.spotColors = true;
                    // Save as
                    app.activeDocument.saveAs(saveFile, psdSaveOptions, true, Extension.LOWERCASE);
                }

                /* Not currently used, a placeholder to swap in/out as needed
                function saveTIFF(saveFile) {
                    tiffSaveOptions = new TiffSaveOptions();
                    tiffSaveOptions.embedColorProfile = true;
                    tiffSaveOptions.byteOrder = ByteOrder.IBM;
                    tiffSaveOptions.transparency = true;
                    // Change layers to false to save without layers
                    tiffSaveOptions.layers = true;
                    tiffSaveOptions.layerCompression = LayerCompression.ZIP;
                    tiffSaveOptions.interleaveChannels = true;
                    tiffSaveOptions.alphaChannels = true;
                    tiffSaveOptions.annotations = true;
                    tiffSaveOptions.spotColors = true;
                    tiffSaveOptions.saveImagePyramid = false;
                    // Image compression = NONE | JPEG | TIFFLZW | TIFFZIP
                    tiffSaveOptions.imageCompression = TIFFEncoding.TIFFLZW;
                    // Save as
                    app.activeDocument.saveAs(saveFile, tiffSaveOptions, true, Extension.LOWERCASE);
                }
                */

                /* Not currently used, a placeholder to swap in/out as needed
                function saveJPEG(saveFile) {
                    jpgSaveOptions = new JPEGSaveOptions();
                    jpgSaveOptions.embedColorProfile = true;
                    jpgSaveOptions.formatOptions = FormatOptions.STANDARDBASELINE;
                    jpgSaveOptions.matte = MatteType.NONE;
                    jpgSaveOptions.quality = 10;
                    // Save as
                    activeDocument.saveAs(saveFile, jpgSaveOptions, true, Extension.LOWERCASE);
                }
                */

                /* Not currently used, a placeholder to swap in/out as needed
                function savePNG(saveFile) {
                    var pngOptions = new PNGSaveOptions();
                    pngOptions.compression = 0; // 0-9
                    pngOptions.interlaced = false;
                    // Save as
                    app.activeDocument.saveAs(saveFile, pngOptions, true, Extension.LOWERCASE);
                }
                */

                function docNameToLayerName() {
                    var layerName = app.activeDocument.name.replace(/\.[^\.]+$/, '');
                    app.activeDocument.activeLayer.name = layerName;
                }

                function removeXMP() {
                    if (!documents.length) return;
                    if (ExternalObject.AdobeXMPScript == undefined) ExternalObject.AdobeXMPScript = new ExternalObject("lib:AdobeXMPScript");
                    var xmp = new XMPMeta(activeDocument.xmpMetadata.rawData);
                    XMPUtils.removeProperties(xmp, "", "", XMPConst.REMOVE_ALL_PROPERTIES);
                    app.activeDocument.xmpMetadata.rawData = xmp.serialize();
                }

            }

            // Restore saved dialogs
            app.displayDialogs = restoreDialogMode;

            // End of script notification
            app.beep();
            alert('Script completed!' + '\n' + fileCounter + ' combined files saved to:' + '\n' + outputFolder.fsName);

            // Open the output folder in the Finder or Explorer
            // outputFolder.execute();

        }());

    } catch (e) {
        // Restore saved dialogs
        app.displayDialogs = restoreDialogMode;
        alert("If you see this message, something went wrong!" + "\r" + e + ' ' + e.line);

    }
}

else {
    alert('Stack 3 Document Sets to Vertically Stacked Layers:' + '\n' + 'Please close all open documents before running this script!');
}

https://prepression.blogspot.com/2017/11/downloading-and-installing-adobe-scripts.html

I'm happy to make changes as needed if further refinement is required.

@HeyoThere 

You will need a script, such as here:

https://community.adobe.com/t5/photoshop-ecosystem-discussions/how-to-automatically-add-two-images-into-layer-in-a-single-psd-or-tiff-then-save/td-p/13894614

Simply change the default setQty variable from 2 to 3.

var setQty = 2;

This will stack one layer over the other in sets of 3 images.

A simple action can be created to make the canvas 300% in height and to move the layers into vertical position:

The script has placeholder code to run a named action set and action before saving.