If he is performing al-’Atmah alone and two Rak’at of it...

If he is performing al-’Atmah alone and two Rak’at of it are complete when he remembers that he has forgotten al-Maghrib, he completes it with one more Rak’at to finish its three Rak’at for al-Maghrib, then performs al-’Atmah thereafter.’”

شهور - Allamah Baqir al-Majlisi (Mirʾāt al-ʿUqūl fī Sharḥ Akhbār Āl al-Rasūl (15/64))