What are the most effective ways to get a flat stomach in 3 months?

More muscle less fat: Even the most toned abdominal muscles will not reveal the abdomen you want if they are buried under a beer gut. Decreasing body fat is a key component of washboard abs. That said, anything that strengthens your core (sit ups, crunches, leg lifts, torsion, etc ) all play a role in building those muscles.

FLAT AB: For a flat abdomen eat healthfully ; be physically active. Calories burned must > calories consumed. To tighten ab's do crunches, reverse crunches, exercise ball crunches, pelvic thrusts, lying leg raises, side bridge ; “bicycling” exercise on your back. Engage in regular aerobic exercise (i.e., running, swimming, nordic track, bicycling, kickboxing, walking, cross country skiing, hiking, etc.).