Is it possible for belly get flatter by not eating for 3 days?

Maybe: You will still have bowel movements if you don't eat, which can flatten your belly slightly, but you still make 10 liters of fluids inside the intestinal tract every day! so not eating is robbing your body of nutrition, which can slow your metabolism, and make it harder to get a shape you want. A more sensible way is 1500 calories, high fiber/protein, no liquid cal., and exercise. Good luck!

A temporary change: Lose weight. For flatter abdomen eat healthy & get physically active. Calories burned > calories consumed. To tighten ab's: crunches, reverse crunches, exercise ball crunches, pelvic thrusts, lying leg raises, side bridge & “bicycling” exercise on back. Engage in regular aerobic exercise (i.e. running, swimming, Nordic track, bicycling, kickboxing, walking, cross country skiing, hiking, etc.)