I had triplets and scar with lack of muscle control. How can I tighten it?

Core exercise/ surg. Your best bet is to undertake a course of core strengthening exercises and do this for 4-6 months. If you are unable to accomplish the goals you desire, surgery may help to repair the residual rectus muscle separation.
Abdominoplasty. I assume you are referring to your abdomen. After triplets, your rectus muscles have probably been separated and the entire abdominal wall stretched out; you may even have a hernia. If this is the case, the only way to return the muscular abdominal wall to its original contour is surgical: an abdominoplasty (tummy tuck.) make sure you are done having children before having this operation.