I have found a way of training the abs as hard as possible in only 3 minutes. It is best to try different abdominal exercises to see which ones you respond best to. Choose 2 exercises and work them back to back (with no rest in between) for 30 seconds each until you have done 3 rounds = 3 minutes.
Here is a 3-minute abs for the gym.
AB Exercise 1
Swiss Ball Crunches
AB Exercise 2
Incline Sit Ups