If you want to script it, you can access the position of a layer in the active composition like so:

app.project.activeItem.layer(i).position.valueAtTime();

where i is the index of the layer (counting from 1). Or you can select the layer and use:

app.project.activeItem.selectedLayers[0]

to return the first selected layer of the active comp

You can open a file to write into with

var f = new File("/URI/for/my/file.csv", "w");
f.write("hello, I'm writing to the file")
f.close();

The URI is the path to the file on your local filesystem, expressed as a URI, e.g. "//C/users/stib/Documents/data.csv" on Windows or "/Users/stib/Documents/data.csv" on mac.

You can loop through the frames of a composition by using

var myComp = app.project.activeItem;
var compLength = myComp.duration / myComp.frameDuration; //length in frames
for (var i =0; i < compLength; i++){
    //do stuff
}

So putting it all together:

var f = new File("/URI/for/my/file", "w");
var myComp = app.project.activeItem;
var myLayer = myComp.layer(1); //could also use myComp.selectedLayers[0]
var p;
var compLength = myComp.duration / myComp.frameDuration; //length in frames
for (var i =0; i < compLength; i++){
     p = app.project.activeItem.layer(i).position.valueAtTime(i * myComp.frameDuration)
     f.write(""+p[0]+","+ p[1] +"," + p[2] + "\n"); //write the data as three numbers separated by commas "x,y,z" plus a line return
}
f.close();

You may have to edit the output file with a text editor like notepad++ or textWrangler to format it exactly as you want.