The Armenian historian Sebeos calls him a son of Davit Mamikonian.
Historians, however, agree that Mushegh was the elder brother of Hamazasp IV and Grigor I Mamikonian, who were both princes of Armenia.
In 636, Mushegh III, at the head of an army of 3000 men, and Grigor, prince of Syunik, at the head of 1000 men, were the Armenian contingent who joined the army of Rostam Farrokhzad, the spahbed of the Sasanian forces, who was preparing to fight the Muslim Arabs who were camping at Qаdisiyyah.
Grigor of Syunik, along with his one son, and Rostam and much of the Sasanian army were also killed in the battle.
[3] In Parvaneh Pourshariati's view, Mushegh may be identifiable with Jalinus, a Sasanian general mentioned in Arabic sources who also died at Qadisiyyah.