Unless the root ball filled the 3 gal. container at the time you planted it in the ground, I'd probably put it back in a 3 gal. container. If you previously had roots starting to circle the container, pot up to a 5 gal. Just be sure to plant into proper container media instead of the soil in which it was planted in the ground.
The right sized pot is one with no more than four radius inches of future root growth. The four-inch rule relates to the growth habit of the root system. Maximum root density is obtained through encouraging branching within the first four inches of a root strand closest to the tip by terminating the tip; however, you will really only obtain maximum root density by using an air-pruning container such as the Root Maker containers advocated by Millet.
Also, deal with root issues in a timely manner. Don't procrastinate as I have done with several trees. Repot or replace media sooner rather than later. Good root health is the key to a healthy tree.
Kent