Why is it necessary to have 3 surgeons in the OR?

An acquaintance told me she had a hysterectomy as well as a small fatty tumor removed. The gynecologist did the hysterectomy, another surgeon did the tumor removal from her abdomen and a third surgeon was on stand by just in case. Couldn't the gynecologist do the tumor removal and/or be the back up for the general surgeon and vice versa? Is it really necessary to have 3 surgeons present and be required to pay all 3?