William III (or in Occitan: Guilhem III) was the Lord of Montpellier from 1025 until his death in 1058. He was the son of William II and husband of Beliardis. His son and successor was William IV. He is the last of the "shadowy" lords of Montpellier, none of whose charters are conserved in the family cartulary, the Liber instrumentorum memorialium.
- Archibald R. Lewis, "The Guillems of Montpellier: A Sociological Appraisal", Viator, 2 (1971), 159.