William III of Narbonne

William III was the nominal Viscount of Narbonne and Judge of Arborea (1407–1420).

He was the grandson of Beatrice, youngest daughter of Marianus IV of Arborea and Timbra de Rocabertí, and Aimery VI of Narbonne (married 1363). When Marianus V, the youngest son of Beatrice' elder sister Eleanor, died in 1407, Arborea experienced a succession crisis. The late Beatrice had a claim to the judgeship which was picked up by her grandson, son of William II.

The real judge from 1407 was Leonardo Cubello, great nephew of Hugh II of Arborea. On 6 October 1408, Martin I of Sicily disembarked at Cagliari with a strong army. On 8 December, William also reached Cagliari. He was crowned "King of Arborea, Count of Goceano, and Viscount of Bas" at Oristano on 13 January 1409.

The two forced, of Martin and of William, met at the Battle of Sanluri. The Arborean troops of William, including many Genoese crossbowmen, broke into two battles. The left battle was destroyed in a location called occidroxiu (the slaughterhouse). The right battle was broken into two, one part chased to Sanluri and there routed and the other forced, under William, to take refuge in the castle of Monreale, in the near village of Sardara, where it resisted. On 4 July, the Villa di Chiesa surrendered to Giovanni di Sena, a serious disaster, even though the Sicilian king had died on 25 July in Cagliari. William returned to France to seek aid, leaving Leonardo Cubello in charge in his absence. Leonardo successfully defended Oristano.

William returned to Sardinia in Spring 1410. He reorganised his territories with his capital at Sassari. With the help of Nicolò Doria, he recaptured Longosardo on 9 August. He tried to take Oristano and Alghero, which were defended by Peter Torrelles, who died of malaria in 1411. On 5 and 6 May 1412, he entered Alghero, but was driven off by the citizenry.

William sold Arborea to Alfonso the Magnanimous on 17 August 1420 for 100,000 gold florins. He died in 1424.