Ermengol or Armengol III (10321065), called el de Barbastro, was the Count of Urgell from 1038 to his death. He was the son of Ermengol II, Count of Urgell and his wife Velasquita "Constança", probably the daughter of Bernard I, Count of Besalú.
Allied with his contemporary and second cousin Ramon Berenguer I, Count of Barcelona, together they shared in the process of erosion of the comital authority to the noblesse. They also cooperated in the Reconquista and Ermengol received a third of the reconquered territory, occupying, in 1050, Camarasa and Cubells after taking them from Yusuf of Lleida. In 10391040, Ermengol and Raymond Berenguer signed a pact against Raymond of Cerdanya. Later in that decade, Raymond Berenguer paid 20,000 solidi for Ermengol's support and military aid.
Ermengol took part in the Barbastro War of 1064 under the banner of his brother-in-law Sancho Ramírez of Aragon. When Barbastro was captured, he was given lordship of the city. He died before 12 April 1065 defending the city from Moorish reprisals and was buried at the Monastery of San Pedro de Àger.
Ermengol married Adelaide before 1048, who died before 1055 and whose family is not known with certainty, though some scholars have considered her the daughter of Guillem I, Count of Besalú. Ermengol and Adelaide were the parents of two children:
Before 7 May 1055, Ermengol took as his second wife Clemencia, hypothesized to have been the daughter of Berengar Raymond I and his second wife Guisla (based on the names of their younger sons), by whom he had three more sons:
Clemencia died sometime after 17 October 1059, when she apparently confirmed a charter with her husband, and before 6 November 1062. Ermengol was remarried to a woman named Elvira, who died before 1063.
In 1063, Ermengol married his fourth wife, Sancha, the daughter of Ramiro I of Aragon.
Ermengol III died in battle near Monzón and his body was first taken to Barbastro and then to the fortress of Àger where he was buried at the entrance of the Church of San Pedro at the Monastery of San Pedro de Àger.