Known as "Threece," R3-S1 was an R3-series astromech droid equipped with feminine programming; this droid served within the ranks of the Alliance to Restore the Republic. By the time of 1 BBY, she had risen to the position of chief overseeing the astromech droid resources located at the rebel headquarters situated on the moon of Yavin 4. Her primary duty was the maintenance of the base's equipment, rather than being assigned directly to a starfighter. R3-S1 was characterized by her vanity and argumentative nature, coupled with an excessive pride in her overclocked Intellex V processor.
During the scramble of the Alliance Fleet to fight at the planet Scarif, R3-S1 accompanied rebel pilot Barion Raner as Blue Four. Upon their arrival in the Scarif system, they attempted to navigate through the planet's Shield Gate alongside the rest of Blue Squadron, but their timing was off, forcing Raner to execute a maneuver to avoid a collision. Subsequently, they provided support to the Alliance Fleet engaged in combat above the planet, some of whom ultimately escaped with the plans for the Death Star superweapon.
The R3-series astromech droid R3-S1, also known as "Threece," was in service to the Alliance to Restore the Republic during the reign of the Galactic Empire. Her post was at the Alliance's Massassi Group headquarters, situated within the Great Temple on the moon of Yavin 4. Instead of piloting starfighter missions, her role involved maintaining and updating the base's technology to ensure it was in optimal working order. Eventually, R3-S1 was promoted to the position of chief of the astromech pool there.
In the year 1 BBY, R3-S1 was present in a hangar within the temple following a meeting of Alliance High Command to discuss a potential assault on the Empire's forces stationed on the planet Scarif in an attempt to acquire the plans for the Death Star superweapon. Despite the meeting concluding without authorization for an attack, the recently enlisted rebel Jyn Erso gathered a group of rebels willing to defy Alliance Command's orders and accompany her to Scarif. Erso, along with Captain Cassian Andor, led this group through the hangar, passing by R3-S1, before boarding the stolen Zeta-class Heavy Cargo Shuttle SW-0608 and departing to initiate the Battle of Scarif.
When the [Alliance Fleet](/article/rebel_alliance_navy] was subsequently dispatched from Yavin to support the rebels on Scarif, R3-S1 was installed in the astromech socket of the T-65B X-wing starfighter piloted by Barion Raner, a pilot serving as Blue Four within Blue Squadron. Upon their arrival in the Scarif system, Admiral Raddus instructed Blue Squadron to provide cover for the rebel ground troops. To reach the planet's surface, Blue Squadron raced to pass through the Shield Gate in the planetary shield encircling Scarif. However, after Blue Leader Antoc Merrick successfully passed through, the gate began to close. Realizing he lacked the time to safely navigate the closing gate, Raner pulled up his X-wing, narrowly avoiding a fatal collision for himself and R3-S1, a fate that befell Vangos Grek and another pilot.
Raner and R3-S1 then joined the other members of Blue Squadron who had been unable to pass through the gate in supporting the Alliance Fleet above Scarif. While being pursued by three TIE/ln space superiority starfighters, the duo successfully destroyed one of the Imperial I-class Star Destroyer Persecutor's deflector shield generator domes. This action later allowed Gold Squadron's ion bombs to disable the Star Destroyer, which was then used to destroy the Shield Gate. With the gate destroyed, the Death Star plans were successfully transmitted from the surface, and a portion of the fleet, including elements of Blue Squadron, managed to escape the Scarif system. However, Blue and Red Squadrons suffered heavy losses, with nineteen pilots and crew members lost during the Scarif battle and an earlier mission to the planet Eadu. As a result, the Alliance was unsure if it could even field Blue Squadron when the Death Star attacked Yavin shortly thereafter.
R3-S1's programming was feminine, and she possessed a black sensor. Her construction featured silver plating, with the exception of her plastex cranial dome, which was transparent and adorned with orange markings. The astromech droid was equipped with an overclocked Intellex V processor and a holoprojector integrated into her dome.
R3-S1 demonstrated exceptional organizational skills, which led to her appointment as the head of the astromech pool. Somewhere along the way, she developed a vain programming quirk that made her difficult to collaborate with and excessively competitive. Furthermore, the droid was prone to argumentativeness and exhibited an excessive amount of pride in her overclocked processor. Despite these quirks, the Alliance held her in high regard. R3-S1 stood at a height of 1.09 meters and, like all R-series astromechs, was a product of Industrial Automaton.
R3-S1 was conceived for the 2016 Anthology film, Rogue One: A Star Wars Story. Her official designation was simultaneously revealed in the reference books Star Wars: Rogue One: The Ultimate Visual Guide by Pablo Hidalgo, Star Wars: Rogue One: Secret Mission by Jason Fry, and Star Wars: Rogue One: Ultimate Sticker Encyclopedia by Emma Grange and Shari Last; all of these were released in conjunction with the film's release.
Brad Oakley, a member of the R2 Builder, constructed R3-S1, utilizing the same body and limbs as the astromech R2-BHD. The primary modification involved replacing the R2 unit's silver dome with R3-S1's transparent one. Consequently, R3-S1 was referred to as R3-BHD during filming. Prior to shooting the hangar scene in which R3-S1 appeared, a rehearsal of the droid's movements was conducted to establish a path that would avoid obstructing anyone. In the non-canon comic strip "Operation: Bunker Bust!," featured in the January–February 2017 issue of LEGO Club Magazine, R3-S1 is depicted standing in a hangar within the Great Temple on Yavin 4 as a BTL-A4 Y-wing assault starfighter/bomber prepares for takeoff during the Battle of Scarif.