Three weeks ago i had knee surgery. the incision appears to be healing well but has become very itchy. what can i do to relieve the itching?

Most likely: A normal healing phenomenon. The skin can itch while healing itself back together. Additionally the absorbable stitches placed under the skin can cause itching as they dissolve.

Dermatitis: Likely a reaction to the prep done at time of surgery or a dressing after. What are you putting on the wound now? I'd ask the surgeon what they suggest you use before trying anything.

Itchy: All normal - always ask your surgeon what or if to put on the area.