3rd Baseman, SS, Catcher & Left Fielder, practice two kinds of plays on ground balls hit to the left fielder by a coach.
3rd Baseman gets down on both knees, which forces him to get his hands out in front of his body, and fields grounders hit by the infield coach.
3rd Baseman, SS, Left Fielder, same play as above but with the coach hitting the ball to short left field and down the left-field line.
Catcher, 3rd Base, 1st Baseman practice the 5-2-3 double play, with the infield coach hitting ground balls to the 3rd baseman, who fields the ball and throw to the catcher, who then goes to 1st base.
2nd Baseman & 3rd Baseman play in on the grass to field hardhit ground balls hit to them by the infield coach.
Left Fielder, same as above, but with the coach hitting the ball to the left side and the 3rd baseman serving as the cut-off man.
If the 3rd baseman has both arms stretched overhead (touchdown), the catcher should throw to 3rd from inside the diamond.
If the catcher throws to him, he must run hard at the runner, staying alert for a call from the 3rd baseman.
3rd Baseman. Breaks to the bag as the catcher receives the ball.
Catcher: As he catches the ball, he takes a quick look at the 3rd baseman. If he sees him signalling touchdown, he will throw to 3rd.
In the ensuing rundown, the defenders have to keep alert for a signal from the 3rd baseman. "Four-four" will let them know that the lead runner is going to the plate.