In Greek mythology, Cerberus is traditionally said to have three heads.
Cerberus, also called the hound of Hades, guarded the entrance to the underworld so the dead could not escape. He was usually described as the offspring of the monstrous Echidna and Typhon, and artists often gave him a serpent’s tail and snakes growing from his body.
The three-headed version became the standard in later Greek and Roman literature. Heracles captured Cerberus alive as the final task in some traditions—or the eleventh in others—bringing him out of Hades as proof of the feat. Virgil and Ovid also describe him as triple-throated or three-necked.
The number was not perfectly consistent in every ancient source. Hesiod described Cerberus as having fifty heads, while Pindar reportedly gave him one hundred. Those older and rarer variants explain why three is a conventional mythological answer rather than an absolute rule across every surviving text.