William III (died after 1037) was the count and margrave of Provence from 1014 to his death. He inherited the titles of his father Rotbold II but preceded his cousin William IV as count.
His mother was Ermengarde, later the second wife of Rudolph III of Burgundy. He is recorded as late as 1032 with the title of marchio and is last recorded donating property to Cluny in 1037. William had no known descendants and he left his margravial rights to his sister, Emma, who married William III Taillefer, Count of Toulouse, and thus brought the margravial title in Provence to the House of Toulouse.
References
Further reading
Lewis, Archibald R. The Development of Southern French and Catalan Society, 718–1050. University of Texas Press: Austin, 1965.