XM-G3, also known as Gee-Three, functioned as a bodyguard droid before retiring from his role as a bodyguard to become a member of commander Lina Graf's Rebel Alliance crew. He took on the role of gunner on Graf's starship, the Auric, and was on board when it sustained damage from TIE fighters during a mission, resulting in a crash landing on the planet of Mustafar. He was later damaged in combat with stormtroopers when the crew made the decision to enter Darth Vader's fortress. While exploring the castle, the Force was used by Vader to tear him asunder, leading to his destruction.
XM-G3, previously a bodyguard droid with extensive operational experience, eventually transitioned away from his duties as a bodyguard. He joined a Rebel Alliance crew under the leadership of Commander Lina Graf, where he served as the crew's strongman. XM-G3's responsibility was that of gunner aboard the Auric, Graf's ship.
The Auric's crew was assigned a fact-finding mission to the Outer Rim by Milo Graf, which led to an attack on the ship by several TIE fighters. As the Auric suffered significant damage, Graf requested that XM-G3 deal with the pursuing TIEs. The droid successfully shot down the fighters, but the Auric was too badly damaged to remain airborne. Spotting a nearby planet, the former mining world of Mustafar, Graf chose to land there. The Auric touched down in a lava field and started to heat up and sink. Despite warnings from CR-8R, the crew exited the ship and watched as it completely disappeared beneath the lava's surface. Upon noticing a fortress in the vicinity, Graf decided to proceed towards it, much to the surprise of her crew.
As Graf's team readied themselves to approach the fortress, Graf instructed them to remove any Rebel Alliance insignia to maintain a low profile. When Lieutenant Thom Hudd voiced his concerns about her plan, she pointed out the larvae that were beginning to crawl on him. Simultaneously, an adult beetle loomed over them. XM-G3 intervened to protect Commander Graf, firing at the beetle. The beetle, wounded, detonated itself, allowing the other larvae to escape. Hesitant about entering the castle, Hudd shared a story with the crew about an event that occurred on Bray during the Clone Wars, which XM-G3 found enjoyable. After Hudd finished, Graf turned her attention back to the castle, attempting to formulate a plan to gain entry. However, before she could, several lava troopers launched an attack on them.
While CR-8R, Hudd, and Skritt ran towards the castle, XM-G3 and Graf defended the crew from the stormtroopers. Graf commanded the bodyguard droid to follow, but he declined, expressing his desire to stay and protect her. He urged Graf to join the others, and she reluctantly complied, leaving XM-G3 to fend off the stormtroopers. Later, XM-G3 defeated the remaining stormtroopers, but he sustained some damage. His plating was cracked and dented, and he had lost one of his arms. Upon entering the fortress, he startled Skritt and informed Graf that the stormtroopers had been eliminated. Despite his injuries, he assured Graf that he was fine and inquired about Lieutenant Hudd's whereabouts.
Upon hearing Hudd's screams, the crew rushed to his aid. They entered the room where Hudd had been, only to find that he had vanished. Deciding to split up to search for him, Graf instructed XM-G3 to stay with Skritt while she partnered with CR-8R. When Skritt expressed his fear, XM-G3 encouraged him to be brave. Unable to lift Skritt's spirits, Gee-Three decided to tell him a story about the Ewoks of Endor. However, the story only amplified Skritt's fear. Rounding a corner, XM-G3 spotted Vaneé, who was contacting his master to report the rebel presence in the castle. XM-G3 prepared to shoot the attendant, who merely laughed and dared him to try. Unbeknownst to the rebel crew, the fortress belonged to Darth Vader, who used the Force to lift XM-G3 into the air. Unable to comprehend what was happening, XM-G3 was torn apart, his remains falling to the floor.
XM-G3, as a bodyguard droid, was programmed for protection and defense, and he possessed skills in operating starship weaponry. He could fire a blaster integrated into his hand without hesitation. He enjoyed combat and demonstrated proficiency in it, managing to survive alone against a group of stormtroopers.
XM-G3 made his debut in the 2018 canon comic Star Wars Adventures: Tales from Vader's Castle 1, authored by Cavan Scott and illustrated by Derek Charm.