Williamson (6-foot-5, 200), who was with the 66ers in the 2010-11 season, spent last year playing for Ehingen in Germany. Two years ago, the St. Joseph's product averaged 3.8 points, 1.8 rebounds and 1.8 assists in 21 games in Tulsa.
Little (6-foot-6, 218) and Justice (5-foot-10, 165) both received training camp invites after being among the 63 players to participate in the open tryouts.
A forward from Kansas, Little was a part of three Big 12 regular-season championship teams, appearing in 55 games in his three seasons.
A guard from Martin Methodist College in Tennessee, Justice is the reigning NCAA Slam Dunk Champion.
Tulsa will add to its training camp roster during the 2012 NBA D-League Draft on Friday at 6 p.m. The 66ers will be granted the sixth pick of the draft and will have eight total selections.