X-3 was a male Human assassin of the Human species. He was one of seven men recruited to take part in Project Omega. This project was an experiment conducted by Commander Rezi Soresh with the goal of engineering the ultimate assassin for the Galactic Empire. To achieve this, Soresh erased their former identities from a computer system and altered their appearances through surgery. He then subjected them to a harsh reconditioning process, intending to transform them into emotionless and memory-less servants.
Initially, X-3 formed bonds of friendship with the other participants in the training, including X-1, X-2, X-4, X-5, X-6, and X-7. However, as Soresh began using electrocution as a method of torture, X-3, along with the others, underwent a complete transformation of his personality and mind, losing all emotional capacity. Circa 10 BBY, the trainees were forced to fight each other to the death, with only one survivor. Although X-3 was Soresh's preferred candidate for victory, X-7 ultimately killed him using a vibroblade, thus earning the position of Soresh's personal assassin.
X-3's character was presented in a flashback within Rebel Force: Target. This book, authored by Alex Wheeler, was the inaugural installment of the Star Wars: Rebel Force series aimed at young readers and released in 2008.