http://genealogics.org/getperson.php?personID=I00120778&tree=LEO

Around 1002, he married Gerberga, daughter of Otto-William, Count of Burgundy, and Ermentrude, Countess of Mâcon and Besançon.

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http://fmg.ac/Projects/MedLands/PROVENCE.htm#_Toc220666442

GUILLAUME [III] de Provence, son of GUILLAUME [II] Comte de Provence et d'Arles & his [second wife Adelais d´Anjou] ([986/87]-1018 before 30 May, bur Abbaye de Montmajour). "Willelmus comes" donated property to Cluny by charter dated 28 Aug [990] signed by "Rodbaldus comes, Adalaix comitissa, Wilelmus comes et filius eius Wilelmus"[295]. "Dominus princeps et marchio istius provinciæ…Willelmus cum coniuge sua…Adelaix et filio suo…Willelmo" restored property to the abbey of Saint-Césaire d´Arles by charter dated 992, subscribed by "Domnus Rotbaldus comes…Willelmus comes filius Rotbaldi et uxor sua Lucia, Wilelmus comes Tolosanus et uxor sua Ema…"[296]. "Pontius…Massiliensis ecclesie pontifex" issued a charter dated 1005 with the consent of "domni Rodhbaldi comitis et domne Adalaizis comitisse, domnique Guillelmi comitis filii eius"[297]. According to Europäische Stammtafeln[298], he was the son of Comte Guillaume by his first wife but the primary source on which this is based has not been identified. It is probably speculative, based on his marriage date, which suggests that he was born earlier than the date of his father's second marriage. In any case, Guillaume's parentage appears to be proved by the charter dated 1005. He succeeded his father in 992 as Comte de Provence.

m ([1002]%29 GERBERGE de Mâcon, daughter of OTHON GUILLAUME Comte de Mâcon [Bourgogne-Comt%C3%A9] & his first wife Ermentrude de Roucy ([985]-[1020/23]). Rodulfus Glaber states that "Willemus…Arelatensis" married one of the daughters of "Willemus, Henrici ducis priuignus, Adalberti Longobardorum ducis filius" & his wife but does not name her[299]. She is named in several charters of Saint-Victor de Marseille. "Wilelmus comes Provincie coniugisque mea Girberga cum filio nostro Wilelmo" donated property to Saint-Victor de Marseille by charter dated 1013[300]. "Adalax comitissa mater Villelmi quondam Provintie comitis et Geriberga eque comitissa…eiusdem principis olim uxor" donated property to Saint-Victor de Marseille for the soul of their late son and husband respectively by charter dated 1018[301]. "Geriberga comitissa" donated property to Saint-Victor de Marseille for the soul of "senioris mei Guilelmi comitis Provincie" and for "filiorumque nostrorum…Wilelmo, Fulcho, Jozfredus" by charter dated 1019[302]. "Gisberga comitissa…cum filiis meis Willelmo, Fulcone Bertranno, Gaufredo" donated property to Saint-André-lès-Avignon by charter dated 1019[303].