What is the difference between myotomy, myomectomy and myectomy?

Different purposes. "myo" refers to muscle. A myotomy is an incision into a muscle such as might be done to cut an overly tight muscle between the esophagus and stomach. A myomectomy is the removal of a myoma which is a benign tumor composed of muscle cells that grows in the uterus. A myectomy refers to removing a overgrown muscle septum in the heart wall.