Trifold size (18cmx18cm)

I would suggest making a document 18cm x 53.7cm. The part that gets folded inside would be 17.7 cm wide, 3mm less that the other two, so that it would fit comfortably.

This layout would be for the inside pages (when opened up). For the back, you would use the same layout, only rotate it 180°, putting the narrow page on the left.

You might have to adjust this on advice from your printer, about the allowance need for the folds.

In your picture of the brochures  I see two brochure folders two different ways and the one standing I see has printing on both surfaces of the printed material. Your diagrams show thee images on a single side if the printed material.

If it is the same printed material folded two different ways it would be a six page brochure printed duplex.  You would to know how the printer handles duplex printing. How the printer prints the second side of the material. Does the print flip the materials or is it done as two different print job. Where you print three images on one side than three images on the second side. As long as the pages have print margins there no need to have spacing between images the folds will be in the print margins. If it were single images the would be like a Playboy centerfold a trifold damsel

arianna.fesani  wrote I have to make a trifold roll with a final size of 18cmx18cm. I have never made a document of this type for printing, but I know that the measurements on each page are slightly different from each other (the cover is bigger?) Moreover, the cutting margins are enough of 3mm? How much space will the folding take up, considering using a thick cardboard? Thanks to anyone who will help me (I understand better the measures in cm than in inches)

You seem to want tp print different size images on thick cardboard of some type that will be folded in three and have final a folded square size of 18cm x 18cm.   Cm and Inches are just unit of measurement Photoshop can handle either  it all come down Print Pixel size.  The size the image pixels will be printed with.  When the print pixel resolution you want to print with you can easily calculate how may pixels your image need to have to print the size you want.

The meaning of  "Moreover, the cutting margins are enough of 3mm" is lost on me I do not understand  that statement.

How much space will the folding take up, considering using a thick cardboard?  I'm sure that would depend on the  cardboard material you will be using.  How it can be scored and compress to fold the way you want.   The three Images will be on one surface of the cardboard.  How do you want the cardboard  folded.  You state the cover.  A trifold is not a book, more a card or brochure  How the folding is done would determine which image is the cover and what orientation pages need to be so they can be read the way you want it to unfold up/down left/right. If you print duplex you can have six pages in your trifold. How would you like them read. If you want six pages you would need to know how the printer prints duplex to know how to layout the page images in you Photoshop document.