Heya theoracle!
I am currently running 4 5gals in a 36x36x72 with just LST and topping. Not too crowded, fits perfect in my opinion. If I were to do another run in the 3x3 I would use 4 or 5 3gal pots. Depends what you're looking for, you could fit 9 3gals in there and do a SOG, stick with 2-5 plants with LST and topping or grow one big 7gal plant or so.
Cheers,