The R3-series astromech droid designated R3-S1 and nicknamed "Threece" served the Alliance to Restore the Republic during the reign of the Galactic Empire. She was stationed at the headquarters of the Alliance's Massassi Group in the Great Temple on the moon Yavin 4. Instead of being assigned to a starfighter, she was employed in the upkeep of technology throughout the base, making sure it was up to date and in prime condition. R3-S1 eventually became recognized as the chief of the astromech pool there.
In the year 0 BBY, R3-S1 was present in a hangar of the temple after Alliance High Command met to discuss attacking the Empire's forces on the planet Scarif in order to obtain plans to the Death Star superweapon. The meeting ended with a decision not to launch an attack, but the recently recruited rebel Jyn Erso assembled a group of rebels willing to follow her to Scarif against the orders of Alliance Command. Erso and Captain Cassian Andor led the group through the hangar and past R3-S1 before boarding the stolen Zeta-class cargo shuttle SW-0608 and taking off to start the Battle of Scarif.
R3-S1 had feminine programming and a black sensor. She was built with silver plating, apart from her plastex cranial dome, which was transparent with orange markings. The astromech was equipped with an overclocked Intellex V processor and a holoprojector built into her dome. R3-S1 excelled at organization, which earned her recognition as head of the astromech pool. At some point she picked up a vain programming flutter that resulted in her not working well with others and being competitive to a fault. The droid was also argumentative and inordinately proud of her overclocked processor. Despite this, she was highly valued by the Alliance. R3-S1 stood 1.09 meters tall and, like all R-series astromechs, was manufactured by Industrial Automaton.