2 or 3 players shoot from 7 spots on the perimeter, inside then outside the arc. The first player shoots from the first spot until he misses, then switches with the next player. Each player shoots twice from each spot inside the 3, then move behind the arc and repeat. The Celtics best is 53 NBA 3s in a row from the left wing.
See Shooting - Villanova 20, 2 miss and out, Two mistakes, Streak, Eastman 3 and done.