Third moves up the line a couple of strides to take away slide opportunity. Pitcher is closest fielder, First backs him up, then Second. Shortstop is closest to third base, Left fielder backs him up on overthrow. With trailing runners, Center and Right fielders move to bags, otherwise all the way home. Stealing Third Primary is Third baseman - get the ball to him first.
Left field is the back-up.
Second is backed up by Center and Right fielders. Catcher moves up the line a couple of strides, with First backing him up. First moves up the path to take away slide, with Pitcher, Catcher and Right backing him up. Third moves off his bag toward second and Left fielder comes in to back-up third.