As with most things in Photoshop there's probably more than one way to do it, but I'll try to explain how I do it. Keep in mind, I'm no Photoshop wiz. I do most of my editing in Lightroom and only use Photoshop for the odd thing Lightroom can't do. In this case, you need to make sure your Canvas Size (under the Edit menu) is big enough to accommodate the three (or whatever number you use) pics plus some extra for the border and spacings. In other words, if your three pics are each 1000 px wide and you want a 10 px wide border and spacing, then your Canvas needs to be 3040 px wide. You may need to play around with this a bit to find the 'look' you want. If you want the background to be black, make sure you make the Canvas extension color 'black' in the Canvas Size dialog box. I would start with the first pic you want in the center of the composite and use that as your base for extending the canvas. Then open the others pics individually and copy/paste them into the base image. They should paste in as separate layers so that you can select them separately and move them around to position them correctly. Make sure you edit the image size of each picture so they are all the same before you copy/paste them into the base. (You may not need to do this but I think it will be easier than adjusting the size later.)
In awe and wonderment (2) by Schtumple.
