I would do 2 boards for a 3x3. But 1 board will run a successful grow, just won’t yeild as much as 2 boards. That’s the reason I suggested getting one now if budget is tight and you can decide after a grow or two if you want to add another.
Up to you either way. If you were going to do two boards now, I would order the 260xw kit. I think it will save you a couple bucks. I can go and look at pricing breakdown if you are contemplating going that way