Two players (A and B) on one side, baseline. One player (C) on the other side of the net, baseline also. They all use only the singles court for this drill.
Either one of the players can put the ball in play with a gentle feed. 
The single player (C) will hit all the balls cross-court. The other players (A and B) will each hit the ball only down the line. 
After about 5 minutes switch roles: C plays each ball down the line and the doubles team hits only cross-court. 
This is an exhausting drill for the single player; good work-out and cardio. 
Players rotate after about 10 minutes so that each one of them will play versus the other two players.

Note: This is a drill for advanced players that requires very good footwork, stamina and ball control.