Wilhelm III De Hornes 
Wilhelm III De's father was Wilhelm II De Hornes and his mother is Margarethe D'Altena de Montbeliard. His paternal grandfather is Wilhelm I De Hornes. He was an only child. He died in 1265.General Notes
SOURCES: LDS FHL Ancestal File # (familysearch.org)
WEB:
"Ancestors/Descendants of Royal Lines" (Contributors: F. L. Jacquier (History of Charlemagne by Christian Settipani); L. Orlandini, Manuel Abranches de Soveral, Reynaud de Paysac, F.L. J P de Palmas (Aurejac et Tournemire; Frankish line; The Complete Peerage}, Jacquier (Genealogy of Lewis Carroll, Justin Swanstrom, The Royal Families of England Scotland & Wales by Burkes Peerage; Debrett's Peerage & Baronage; Table of descendants French Canadian Genealogical Society; Families of Monfort-sur-Risle & Bertrand de Bricquebec; The Dukes of Normandy, XXXXI), A. Brabant ("Dynastie Montmorency, Michel d'Herbigny), Paul Leportier, Claude Barret, H.R. Moser (Burke Peerage), O.Guionneau, L.B. de Rouge, E. Polti, N. Danican (Britain's Royal Families; Buthlaw, Succession of Strathclyde, the Armorial 1961-62) A.Terlinden (Genealogy of the existing British Peerage, 1842), L. Gustavsson, C. Cheneaux, E. Lodge, S. Bontron (Brian Tompsett), R. Dewkinandan, H. de la Villarmois, C. Donadello; Scevole de Livonniere, H. de la Villarmois, I. Flatmoen, P. Ract Madoux (History of Morhange; Leon Maujean; Annuaire de Lorraine, 1926; La Galissonniere: Elections d'Arques et Rouen), Jean de Villoutreys (ref: Georges Poull), E. Wilkerson-Theaux (Laura Little), O. Auffray, A. Brabant (Genealogy of Chauvigny of Blot from "Chanoine Prevost Archiviste du Diocese de Troyes Union Typographique Domois Cote-d'Or 1925), Emmanuel Arminjon (E Levi-Provencal Histoire de l'Espagne Andalouse), Y. Gazagnes-Gazanhe, R. Sekulovich and J.P. de Palmas ("notes pierfit et iconographie Insecula", Tournemire), H de Riberolles (Base Tournemire), Franck Veillon; ,(Histoire Généalogique de la Maison de Hornes, Bruxelles 1848; Notice Historique Sur L'Ancien Comté de Hornes, Gand 1850; Europäische Stammtafeln, Marburg 1978).
........... http://geneastar.org.
They had a son named Wilhelm IV De.