In the above example, the tri-set I was referring to was something I' ve heard other uses effectively to reduce the time in the gym.
Basically you do one ab exercise for 15-20 reps, then move to another for 15-20 reps and finish of with another 15-20 of a different exercise.
So you did 3 different ab exercises, with little to no rest between the sets. You do a certain number of exercises and reps without taking a break.
You' ve just done 3 sets of 15-20 reps by just moving from Exercise A, to B, to C.
You might do
1) Incline crunches
2) Leg raises
3) Side crunches for the obliques
Personally, I do a set.. high reps.. rest.. do it again.. rest.. do it again.. so I do my 3 sets (maybe different exercises) but not all at once. And I do it at the end of my workouts so that my abs (stabilizer muscles) are not fatigued when I' m working my primary bodypart of the day.
I just recently started working abs. They are strong but I' ve never actually done any exercises for them specifically. The weighted incline crunches are enough to do me in.