I had ACL reconstruction surgery using a hamstring graft 3 months ago and now I'm having sharp pain in my hamstring. Any suggestions?

ACL. Not uncommon. If acute onset, usually ice and rest followed by rehab of hs with stretching and strengthening is all that is usually required. Would recommend discussing and having evaluated by your surgeon.
Therapy can. Help certainly. Often times the hamstring harvest site can get some scarring that can irritate your thigh. Stretching and nsaids help as well.