William III , Chatelain de St. Omer St. Omer

William III , Chatelain de St. Omer's father was William II , Chatelain de St. Omer St. Omer and his mother was Melisinde De Pecguigny.  His paternal grandparents were William I Chatelain de St. Omer and Aganitrude Von Brugge; his maternal grandfather was Arnoul De Pecguigny. He had a sister named Maude.  He died in 1170.

They had a son named William IV Chatelain de , Seigneur de Fa.