From my experience, you *might* be able to get away with 3 gallon buckets if (and this is quite hypothetical):
1. Veg for 3 weeks and flower for 7-8
2. Repot at least 4 times (letting the roots bunch up a little before transplanting, but not to the point of totally rootbound), until reaching the 3 gallon buckets sometime in early to mid flowering.

Of course your growing conditions will have an effect also (i.e. plant genetics/lumens/temperatures/soil composition/fresh air/nutrients) Many things to consider. For me, I end in 15L pots, this is with about 4 weeks veg and 9 weeks flowering under a 400w hps, and not a 'high yielding' strain.

You can always try with the 3 gallons and if they get too big, go get some 4-5 gallon jugs. Just make sure you have enough room for the larger pots in case you need them. Transplanting in flowering has not been a problem for me (nor anyone else as far as I know)

Some more info here: http://boards.cannabis.com/basic-gro...ency-baby.html

:twocents:

Happy growing :jointsmile: