1 FT of plant growth per gallon is a good rule of thumb, and it sounds like you will flip them pretty soon, so in that case I would keep them in the 3 gallons, but someone else may differ. The thing is, a bigger pot that your roots may not come close to filling up may be to big and stunt growth a little as well. Roots in dirt kind of like to huddle together and when they have allot of room in soil they can consentrate so much energey to making that bunched up root ball that it takes away from some of the energey that it can focus above the soil to the top.
I no a fellow that gets better yields in 2 gallons than 3 with 3 weeks veg and a dialed in strain he pulls 2 OZ's from every plant, but when he used the 3 gallon it just didn't click the same way, so bigger is not always better.
Adieu