They shared one eye and one tooth, which they took turns using. By stealing their eye, the hero Perseus forced them to tell the whereabouts of the Medusa, so that he may behead her.
In Greek mythology these sisters were the daughters of the sea deities Phorcys and Ceto and are related to the Gorgons. The three sisters usually were referred to as one and had many names; Graeae, Grey Witches, Grey Sisters, Phorcides and more.
Image © Jacob Walker