From Halopedia, the Halo wiki
Fall of Reach
1 Alpha 3 was the call sign of a UNSC Army trooper from 1 Alpha who fought on Reach during the fall of the planet in 2552. During the Siege of New Alexandria on August 23, the trooper and her unit were pinned down on a rooftop by Brutes; however, SPARTAN-B312 was able to assist them. She and the rest of her squad were then picked up by a Pelican.