Is it possible that a post hysterectomy patient could have a 3rd ovary that was not found?

Unlikely: There is one report of the presence of a 3rd ovary in the medical literature from 1880. This ovary was accompanied by a fallopian tube which was attached to the uterus. In the unlikely event that a 3rd fallopian tube was present the gynecologic surgeon would go looking for an additional ovary.

Possible: While it is extremely uncommon for a woman to have three ovaries it is possible. More likely, when the ovaries were removed during the hysterectomy, a small portion of one of the ovaries, a remnant, was left behind.