I tried to create an image in photoshop with 9 equal squares and I failed horribly. Is there a simple way to divide a canvas into equal sizes rectangles without having to work out the numbers?
Yes. You can use guideguide . It’s a quite simple extension that you can install on photoshop. From then on just make columns(3) and lines(3) (in your case) and you have your 9 spaces in your canvas.
Hope it helps.
Curently PS CC 2017 as a “New Guide Layout” under
View > New Guide Layout, that allows to do the same as guideguide, though not so fast.