The Triple Killer, nicknamed 3XK, is a notorious serial killer who, beginning roughly four years before Castle and Beckett begin their partnership, killed six women then mysteriously disappeared. He got his name because he killed in threes. All his victims were strangled with a white and green rope cord, and were young, blonde women who he would arrange in a funereal posture, with hands crossed on their chests.
In 3XK (episode 3.6), Castle and Beckett discover he has returned when they are assigned to investigate the murder of a young blonde woman found in an alley. Another woman dies, also young and blonde and the two, along with Detectives Javier Esposito, Kevin Ryan, and Captain Roy Montgomery, who served on the initial task force that investigated the crimes, race against time to identify the killer and stop him before he claims his third victim. They think they accomplish just that, only to find that the real Triple Killer outsmarted all them all and escaped after holding Castle and Ryan hostage at gunpoint.
Castle profiles the killer as having come from a single parent household. His mother, a blonde woman, never wanted him, and died when he was around 12 years old, landing him in bad foster care settings. He takes out his hatred towards his mother on the women he kills. When Castle tells him that he knows his face and his name, the killer merely says that he'll change his face and start over somewhere new, just as he's done before.
Executive producer Andrew W. Marlowe said on February 12, 2012: "We're having talks about whether we're bringing him [3XK] back by the end of this season [Season 4] ... But we're still waiting to hear on Season 5. If we get the Season 5 pickup, it's something that we might wait and make more of a meal of next year."
The character returns in the season 5 episode Probable Cause where he frames Castle for murder, but thanks to the hard work of Beckett and her team, he is cleared. As Beckett drives him back to the precinct, the killer intercepts them on a drawbridge, and holds Beckett at gunpoint until Castle shoots him and 3XK falls off the bridge. While Beckett believes he is dead, despite the fact that his body is never found, Castle refuses to do so, believing 3XK planned the murder and his framing all so he could fake his death and continue killing without worry of anyone looking for him.
3XK resurfaces in Disciple when he recruits a security guard, Carl Matthews, to kill two people who received plastic surgery to look exactly like Detective Javier Esposito and Lanie Parish all so the look-a-likes can steal all evidence and files related to 3XK's previous murders. Castle and Beckett also discover he has enlisted the help of a well-known and respected plastic surgeon, Dr. Kelly Neiman.
In the season 7 two-parter, Resurrection and Reckoning, Nieman and 3XK concoct an elaborate plot to kidnap and torture Beckett in front of Castle while having apparently unshakeable alibis: Nieman cannot be tied to the crime while 3XK passes himself off successfully as a man called Michael Boudreau, and cannot be arrested. 3XK lures Castle, or so he thinks, to a remote house where he is to watch Dr Nieman torture Beckett on the operating table. But Castle has anticipated this and set Esposito up as a sniper outside. Castle lures 3XK to the window, and Esposito shoots 3XK. They race to save Beckett, to find that she has already escaped from the operating table and killed Nieman.
3XK is played by actor Michael Mosley.
Visit the Linda Russo Murder Board to see how the case unfolded.