I've got mine (outdoor) in 5 gal., they ended up being about 4 feet tall, not counting the pot (like we're supposed to measure
), and they still have plenty of room.
One is an Indica-ish, and the other is a more Sativa-ish, so I think 3 would be fine.
Watch for Scrog to see if he chimes in, I've found his comments and views to be helpful.
Note:
Last night, I pulled the top 6" or so off my Indica, and got 18.3g wet, but weighing tonight, I have already lost 10.1g due to water weight.
I still have about as much to pull once it fills in a little and ripens, but my Sativa will have lots more buds, but looks like it's going to be a lot fluffier.