Castle Season 3 Episode 6 (Castle s03e06) with title “3XK” aired on Monday, October 25th 2010 at 10:00 pm via ABC Network.
Here is summary for Castle Season 3 Episode 6 :

When a beautiful woman is found dead in an alley, all clues point to her being a victim of the notorious Triple Killer. The Triple Killer, or 3XK, terrorized New York City four years ago, then mysteriously disappeared. Now he’s back. In a deadly battle of wits, Castle and Beckett race against time to stop 3XK before he claims his next victims.