John III, Duke of Brittany

John III, also known as John III the Good, was born on 8 March 1286. He became Duke of Brittany in 1312, following the death of his father, Arthur II. John was opposed to his father's second marriage to Yolande and attempted to contest its legality. In 1297, he married Isabella of Valois, but she died childless in 1309. He then married Isabella of Castile in 1310, who also died without children in 1328.

In 1329, John married his third wife, Joan of Savoy, but this marriage also produced no heirs. John was unwilling to pass the Duchy of Brittany to his half-brother, John of Montfort, due to his dislike for his stepmother Yolande. He intended to leave the duchy to King Philip VI of France, but his nobles objected. The marriage of his niece Joanna of Dreux to Charles of Blois provided Charles with a claim to the duchy, which remained unresolved at John's death.

John III died on 30 April 1341 at the age of 55 and was buried at Notre-Dame des Carmes. His death led to a succession dispute between John of Montfort and Charles of Blois, triggering the Breton War of Succession.