I had 2 natural births and now had my 3rd child via c-section last year. Since have had trouble losing stomach fat. Suggestions on how to tone again?

There's no such . Thing as natural spot fat loss. You need to lose weight in general ; a certain % will be abdominal fat. 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.
OBESITY. With 3 kids to take care you need to give time to exercise. Try bending and other abs exercise like dancing, walking a lot, gardening accompanied by less food or less calories like fruits and veggies.