Why would a physician advise not to do any light exercises 3 wks after an abdominal myomectomy even though i feel fine and have no pain or soreness??

Wound healing: Even with "superglue" for wound closures, there are several layers under the skin that have been surgically repaired that the body needs time to heal. The most important layer is the fascia, which if it "comes apart" prematurely will mean the entire wound itself will do likewise. Exercises put a stress on the wound closure as the muscles tend to try to "pull apart" any wound closure, so heed MD.