You are going to need to describe how you currently attempt to make the montage. PPI makes no difference, the document is working with pixel dimensions not physical dimensions for printing.
If I make a new document that is 3000px x 600px and 300 pixels per inch, or 150 pixels per inch the result of pasting a 600px x 600px image into each new document is identical. The difference will be in the physical size of the printed document.
I suggest you try the following:
1) Make a new document, 3000px x 600px at the DPI res that will give you the final print dimensions you need - if you do not plan on printing the document, DPI is irrelevant.
2) Crop each of your 5 images so that they are 600px x 600px. I do not know what size your images start with, but you will likely have to select or crop a square area to get the 1:1 aspect ratio - to fit the entire image into a 600x600px crop, you may need to resize the original image prior to cropping or selecting. In the Image Size dialog box, select *pixels* as the dimensions for the Width and Height input boxes and make sure Resample is checked.
An easy way to make a 600x600 px selection is to use the marquee tool and set the style to "Fixed Size" - type in 600px for the width and height of the selection. Then, when you go to make a selection with the marquee tool, all you have to do is click on the image and a 600x600 pixel marching ant box will be created - you can move it around to select the area of the donor image that you want to use for the montage.
3) Copy the selection to the clipboard and paste into the new, blank 3000x600 document. It will automatically be aligned in the center of the document, on a new layer. You can reposition this image by making its layer active and using the move tool to move it left or right - holding SHIFT while moving will constrain the movement to the initial move direction - i.e., horizontal - this will make sure you do not move the layer out of vertical alignment off of the document.
Repeat.
You can always align the content of selected layers by selecting the layers in the layer panel and using the align choices (align and distribute).
There are much easier, automated ways, but the problem you are describing needs to be troubleshot manually before you can start automating things. If you do all of the above correctly, and you see "gaps" at the edges of the document or between images and the edge of the document, try flattening the document - the small "gaps" should disappear.
kirk

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