In 1391, he had to leave for Italy in order to go to the assistance of Charles Visconti, Lord of Parma and husband of his sister, Beatrice of Armagnac. Visconti was in conflict with his acquisitive cousin Gian Galeazzo Visconti, later the duke of Milan, whose ambition was to control the whole of northern Italy.
On May 14, 1378, John III married Margaret (1363–1443), countess of Comminges (1363–1443). They had two daughters:
- Joan, who married Guillaume-Amanieu de Madaillan (1375–1414) in 1409.
- Margaret, who married William II of Narbonne in 1415. He was killed in the battle of Verneuil, on August 14, 1424).
- Allmand, C. T.; Coopland, George William (1976). War, Literature, and Politics in the Late Middle Ages. Liverpool University Press.
- Lodge, Eleanor C. (1926). Gascony under English Rule. Methuen & Co. Ltd.
| Count of Armagnac