List of rulers of Brittany
House of Montfort

Name
Portrait
Birth
Marriages
Death
John IV the Conqueror (Yann IV) 1364-1399 (Previously John V in pretentious succession from his father.)
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c. 1339 only son of John of Montfort and Joanna of Flanders
(1) Mary Plantagenet of England Woodstock Palace, Woodstock, Oxfordshire summer of 1361 no issues (2) Joan Holland London May 1366 no issues (3) Joan of Navarre Saillé-près-Guérande 2 October 1386 nine children
1/2 November 1399 Nantes aged 59–60
John V the Wise (Yann V ar Fur) 1399–1442
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24 December 1389 Château de l'Hermine, Vannes, Morbihan eldest son of John IV and Joan of Navarre
Joan of France Hôtel de Saint-Pol, Paris 19 September 1396 seven children
29 August 1442 Manoir de La Touche, Nantes aged 52
Francis I the Well-Loved (Frañsez Iañ) 1442–1450
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11 May 1414 Vannes eldest son of John V and Joan of France
(1) Yolande of Anjou Amboise 20 August 1431 one son (2) Isabella of Scotland Château d'Auray 30 October 1442 two daughter
17 July 1450 Château de l'Hermine, Vannes, Morbihan aged 36
Peter II the Simple (Pêr II) 1450–1457
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7 July 1418 Nantes second son of John V and Joan of France
Françoise d'Amboise c. 1442 no issues
22 September 1457 Nantes aged 41
Arthur III the Justicier (Arzhur III) 1457–1458
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24 August 1393 Château de Suscinio, Vannes second son of John IV and Joan of Navarre
(1) Margaret of Burgundy Dijon 10 October 1423 no issues (2) Jeanne d'Albret Nérac 29 August 1442 no issues (3) Catherine of Saint Pol 2 July 1445 no issues
26 December 1458 Nantes aged 65
Francis II (Frañsez II) 1458–1488
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23 June 1433 Château de Clisson eldest son of Richard de Dreux, Count of Étampes and Marguerite d'Orléans, Countess of Vertus
(1) Marguerite of Brittany Château de l'Hermine 16 November 1455 one son (2) Marguerite of Foix Château de Clisson 27 June 1474 two daughters
9 September 1488 Couëron aged 55
Anne (Anna) 1488–1514
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25 January 1477 Château de Nantes eldest daughter of Francis II and Margaret of Foix
(1) Maximilian I, Holy Roman Emperor (by proxy) Rennes Cathedral 19 December 1490 no issues (2) Charles VIII of France Château de Langeais 19 December 1491 four children (3) Louis XII of France Château de Nantes 8 January 1499 four children
9 January 1514 Château de Blois aged 36

Question: When and how long did Arthur III become the duke of Brittany?
Answer: Arthur III became duke of Brittany for a year, from 1457 to his death in 1458.