Pot size??

In my opinion no, you want at least 21 litres/5 gallon pots for healthy root development in soil, preferably more. As AsNoriu said; - bigger pots = bigger roots = bigger plants. After 5-6 weeks of veg in a 3 gallon pot, your plant would be absolutely root bound, leaving very little opportunity for growth during flowering. Also, with little pots and the high risk of stunting, the small amount of soil dries out very quickly and also has an increased risk of nutrient salts building up to toxic levels and harming your plant beyond repair. I started with 11 litre/nearly 3 gallon air pruning pots/rocket pots for autos and soon discovered their total unsuitability for this task due to very small plants and moved to 21 litre air pruning pots quite smartly, with MUCH better results. If I were to ever grow autos again, 21 litre/5 gallon would be my minimum size and I would probably prefer even larger ones. 21 litre/5 gallon pots would be the absolute minimum for photoperiod plants if you want a decent harvest, but again, bigger would be better, unless you want to grow "sea of green" style and only have a 4 week veg time, then 21 litres/5 gallons would be adequate. As they say in the automobile world "there ain't no substitute for cubes!" Hope this helps,........... Organoman.

I presume we speak about soil, in that case : Small pot will bring you challenges of watering in time, small amount of soil is very unforgiven for young growers mistake, don't make a solo cup challenge from your first grow. I run with 20 liters airpots for autos, but after this run ill go to 30. Hope it helps to make your mind. Bigger pot - bigger roots - bigger plant !

3 gal will work fine for an autoflower. If you do a 6 week veg with a photoperiod I would air prune the pot. I would have the auto pot air pruned also. Con yield could be smaller than a 5 gal pot.

3 Gal = 11-12l pots Yes , they are enough , at least for me , My pots are 11l and left them around 6weeks in veg , 3rd week in flowering and don't seem to have issues with them(other than some cal-mag , but not related to roots or pot size) . Using soil

Yes I grow autos in 3 gallons and they can go over that, so you'll be fine with the 3 gallons.