Is there anyway to stop sphincter muscle from contracting 3 weeks after knife piles surgery?

Should not be a prob: If this is a sphincterotomy or sphincter repair topical nitroglycerine (rective or topical ditilizam should help.

Yes: Avoid constipation at all costs. Drink plenty of fluids and eat a lot of fiber rich foods like bran cereral, quinoa, whole grains, leafy vegetables, brown rice as examples. After anal surgery fear of pain on bowel movements is a major concern for patients so they try to avoid them. Contrary, good bowel movements help to open the sphincter muscle normally thus avoiding muscle spasm and pain.