He succeeded his father while still an infant and was put under the guardianship of John, son of William II, Earl of Ross who wrote to Edward II of England to say that William of Sutherland did not have the experience to govern his earldom.

In the same letter, the Earl of Ross tells King Edward that Robert the Bruce had advanced north with a large army which he and the other supporters of Edward were unable to withstand and so requested the fealty of the earldom of Sutherland.

[2] The young William, 3rd Earl of Sutherland was present at the parliament held at St Andrews on 16 March 1308-09 where the nobles and barons of Scotland wrote to Philip V of France who had asked for assistance in his crusade against the Saracens.

[2] According to 19th-century historian William Fraser the statement of 17th-century historian Sir Robert Gordon, 1st Baronet that the Earl of Sutherland fought for the Bruce against the English at the Battle of Bannockburn probably refers to the 3rd Earl but there is no contemporary record of this.

[2] William was succeeded by his brother, Kenneth de Moravia, 4th Earl of Sutherland.