The three sisters who shared a single eye and a single tooth were the Graeae of Greek mythology.
The Graeae were ancient, gray-haired daughters of the sea gods Phorcys and Ceto, and sisters of the Gorgons. Their name means “old women” or “gray ones,” reflecting their supernatural age and appearance. The best-known names are Deino, Enyo, and Pemphredo, although ancient authors do not agree on the exact number or names.
The sisters took turns using their one eye and one tooth. In the story of Perseus, the hero seized the shared eye as they passed it between them. He used it as leverage to make them reveal information needed to find Medusa, often including the location of the nymphs who possessed his magical equipment.
They are sometimes confused with the Gorgons because both groups were daughters of Phorcys and Ceto. The Gorgons were the monsters Perseus ultimately confronted; the Graeae were their elderly sisters and reluctant informants.