Ruffy, first, welcome to the forum. Good to see you here, man.
OK . . about pots. Can't really say too much because I don't know if you're talking indoor, GH, or outdoor.
Indoor of course you're constrained by lumens, not space, so much, and so pot size is usually not the limiting factor. When I run my GR I'm using 2 x 150HPS and 1 x 150 CMH, and I can run max 6 plants in there with 3 gallon pots, roughly 2 oz per plant yield.
In the GH, well, I run 5 gallon pots and I put the seed straight into the coco. (I have grown in soil too, and I just don't have the patience for that any more.) Why hold the plant back? MJ puts a tap root straight down as fast as it can, once it hits the bottom it starts to circle and the plant goes "oh oh, I have almost no space to grow . . ." - much better to go straight to final size.
Outdoor, well, if in pots, it depends ultimately on your privacy/risk factor. I can run 6oz from a 5 gallon pot outdoor with a 7 ft sativa, or 4 oz in the GH with the same pot and 10 weeks on an indica.
Finally, 3 gallons is plenty for a short-shot auto. If you can find a reliable auto, that is . . . but that's a different story.