Can you wear make up over a three week old surgery scar?

Yes & no. How long ago the surgery was is irrelevant - what you need to focus on is your body & whether or not the scar is COMPLETELY healed. You want the skin to heal first. Do not put makeup over or around the scar if it is not healed, puffy, sore, etc.

If you have stitches, you should wait two or three days after they're removed. You do not want to wear makeup over an unhealed scar because you want to prevent any makeup from getting into stitch depressions, causing self-tattooing.