No matter what I do, I'll come to the same end—termination.
In the years leading to the Blockade of Naboo he found himself in the service of Darth Maul, who severely modified the protocol droid to become a security droid for Scimitar, Maul's starship. Maul built 83 different weapons into the droid's chassis and improved his sensor package. After a successful mission, Maul lost the droid into the demolished Bartokk fortress, where he remained, partially dismantled. However, Maul eventually recovered the droid, and 3PX was seized along with the Scimitar when Maul was killed in the Invasion of Naboo. Fearful of the secrets the droid could reveal, Palpatine had its memory wiped and remanded him to Raith Sienar.
Years later he was bought by Olag Greck and, after being left behind, finally found his freedom, becoming an infamous assassin droid.
When C-3PO was pitted to a death fight in a droid arena, C-3PX took pity on his fellow droid and let himself be destroyed in Threepio's place. When he was being brought to the scrap heap, IG-13 took his head and rebuilt him from the ground up. The droid joined IG-13's party of droid renegades, participating in many missions even up to the Yuuzhan Vong invasion.