X3-51 is a character mentioned in Fallout 4.
X3-51 is a synth courser working for the Institute in 2287. They were tasked to retrieve K8-23, a runaway synth. Instead, they found their lifeless carcass near the Saugus Ironworks, which one could assume they were shot to death by the residing Forged.
X3-51 is mentioned only in Fallout 4.