@Fluffyhead22
All these drivers will work...
Bare minimum for the 3'x 3'...
($115) 320h-54a
Will need to dim or raise height for a 3' x 3'...
($145) 480h-54a
Will need to dim for a 3' x 3'...
($200) 600h-54a
Others are recommending a "b" style driver which would require you to wire in an extra pot. The added pot is nice, but not necessary. This is also true with a driver bigger than a 320h (for your 3'x 3'), they are nice and give options for future changes, but not necessay.
A 480h will give you limited future options as its slightly underpowered to run a 4'x 4' area (but could do it, would run hot and would have to max it) and that seems to be your next tent size up. So for an added $30 (480h over the 320h) you're pretty much getting a driver that's not going to work as hard and possibly give you the ability to flower an additional 2'x 2' if you bought another light to go along with it. (3 lights in the 3'x 3', 1 light in the 2' x 2').
The more I think about it, if you got the cash go even a size bigger, go for the 600h (but it'll cost twice as much as a 320h). The 600h will give you all the versatility I was talking about, and would be able to move up to a 4'x 4' tent with more ease than the 480h.
Ultimately I'm just trying to give you some info so you can make your own decision, personally if on a budget I'd compromise and go with the 480h (only $30 more than the 320h, but not as much as the 600h) but if I had the funds I'd swing for the 600h. If cost is paramount I'd go with the 320h.
No matter the driver size you go with, 320h, 480h, 600h, they all need to be either
54a (built in dimmer) or
54b (wires left hanging off driver to add a pot yourself), they all need to be 54v. Good luck!