John III the Good 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.
Isabella died childless in 1309. Isabella died childless in 1328. 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.
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.