John III, Duke of Brittany

John III the Good (in Breton Yann III, in French Jean III; 8 March 1286 – 30 April 1341) was duke of Brittany, from 1312 to his death and 5th Earl of Richmond from 1334 to his death. He was the son of Duke Arthur II and Mary of Limoges, his first wife. John was strongly opposed to his father's second marriage to Yolande of Dreux, Queen of Scotland and attempted to contest its legality.

In 1297, John was married to his first wife Isabella of Valois, eldest daughter of Charles of Valois and his first wife Margaret of Naples. At the time of their marriage John was eleven years old and his bride five. Isabella died childless in 1309. In 1310, John married his second wife, Isabella of Castile. Isabella died childless in 1328.

In 1329, John married his third wife, Joan of Savoy. He predeceased his third wife by three years and died childless. He was unwilling to cede the Duchy to his half-brother, John of Montfort, son of his hated step-mother Yolande. He wished to leave the Duchy to the French king, but his nobles objected. The marriage of his niece Joanna of Dreux to Charles of Blois gave Charles a plausible claim to the Duchy, but the matter was unresolved at John's death.

After his death, John de Montfort claimed his rights as duke, but the French king supported the Blois faction, and the Breton War of Succession (1341–1364) was triggered. The Breton Civil War was fought between the House of Blois and the House of Montfort. It became part of the Hundred Years' War, as England supported the Montfort faction, which won, against the House of Blois, which was supported by France.