I use 4'', but 3 is ok too.
And going the tub route will save space as well. It's not as hulking as a tank.
As another fellow poor college student, I can't emphasize how much I hate tanks. I had one before tubs and it was a pain. Heating it was horrible. Cleaning it was terrible. Overall, not worth it. It was bad for me and the animal, so I was happy to ditch it. Tubs are much more easy to clean too.
Very smart choice going with the tub set up! When I started out I did the progression from tank, to PVC cage to rack system. I don't even want to know how much $$ I put into the economy learning my lessons.
The tub will be awesome until you secure your T-8.
Thomas "Slim" Whitman
Never Met A Ball Python I Didn't Like