Constantine III was a legendary king of the Britons, as recounted by Geoffrey of Monmouth. He was the son of Cador of Cornwall, a relative of King Arthur. Constantine fought in the Battle of Camlann and was apparently one of the few survivors. Arthur, about to be taken to Avalon, passed the crown to him.
Geoffrey says that Constantine continued to have trouble from the Saxons and from the two sons of Mordred. Those sons were Melehan and Melou. He eventually subdued his enemies, however, and chased Mordred's sons into churches where he murdered them. According to Geoffrey, he was struck down by God for killing them while in sanctuary, and was buried next to Uther Pendragon at Stonehenge.
Though mostly forgotten in later continental romances, the British retained some knowledge of him. He appears, for example, in the Alliterative Morte Arthure and Malory's Morte d'Arthur as Arthur's cousin and successor. One legend identifies him with King Constantine of Dumnonia, who ended his wicked ways and became a saint.