I agree that all the baggies, cups, sphagnum, etc, can be a waste of time, depending on your circumstances. Certainly a 3 gallon pot will work. However, I like using a 1 gallon pot, rather than a 3 gallon, for the following reasons:
1. For rooting, I like a mix of 50/50 perlite and potting mix. But once I transplant to the 3 gallon pot I like to switch over to a mix of mostly pine bark, with a touch of lime, fertilizer, perlite, and possibly peat moss thrown in. The perlite/potting mix can get soggy if you have a large 3 gallon container full of it.
2. I may move my cuttings around a bit while they're young. and I like to cover with plastic bags. It's easier to move and cover the smaller pots.
3. The rootball may end up being better if you start in a 1 gallon pot, then move to the larger. Most roots will grow outward or downward until they hit something. If you start in a 1 gallon pot, you'll get a small, compact rootball, then when you pot up you can decide if this is ok, or you can try to spread it out. If you start in a 3 gallon pot, you may end up with most of your root mass towards the outside of the pot, making it harder to work with.
Just my thoughts.