You are exercising your abdominal muscles, so you are exercising your abdomen; therefore, it's ab exercises. My nephew is a gym rat, and that is the term he uses. ;-)

I would say 'ab exercises' - because (a) you can use 'ab' to mean abdominal, being an adjective that specifies the type of exercise, and (b) because it flows better.

"Abs exercises” because you are (presumably) exercising all the abs, not just one.