I have found a way of training the abs as hard as possible in only 3 minutes. For best results try different abdominal exercises to see which ones work best for you.
How it works
Choose 2 exercises.
Perform the first exercise for 30 seconds.
After 30 seconds swap over to the second exercise (no rest).
Perform the second exercise for 30 seconds.
After thirty seconds go back to the first exercise (no rest).
Perform the first exercise again for 30 seconds and repeat the sequence until you have done 3 rounds = 3 minutes.