Hi Yijaha - they are more or less the same, interchangeable. Stomach is the more formal word. I'm not sure how these words would be used to describe a gym exercise, but stomach is probably the best to use. You exercise your abdominal muscles (or "abs", as in "ab crunches"), which you could also call (less technically) stomach muscles or tummy muscles (informal). You would probably not use "belly" when describing a gym exercise.

I'd go with Alex's suggestion of "abs". The word "stomach" can also mean that part of your digestive system, and even though it's a common word, it has a slight "medical" sound to it. Belly and tummy are colloquial. You would use these in a daily, family/friends context. You would even say these words to little children ("tummy" is baby-talk for "stomach"). "Belly" has the suggestion of a full, round stomach. Checking the etymology of each word would give you some clue as to what word is appropriate in a given situation.