If you want you can add a third defender line at the mid-court similar to what you do in 3 on 2 continuity. Focus on techniques to stop the dribbler and discourage penetration of the defenders near the basket. While you can get more repetitions by practicing 3 on 2's from the mid-court line, I think it's more realistic to practice skills at one end and make them sprint to defensive transition the full length of the court which is more realistic in terms of game tempo. The first post player back should always pick up the first post offensive post player in the attacking court. Often teams break down in transition because one post player fails to cover for another.
Post players are notoriously slow in getting back.
Having them practice communicating and covering for each other will improve your defensive transition. Make one of the post players touch the backboard, or baseline before getting back. When a teammate has picked up the player you are guarding in transition, immediately locate his player or another free player who is in scoring position. When in the zone offense, often a shooter or scoring runs the baseline and is in a poorer position to get back quickly. This results in someone else having to pick up his man.