ABC announced that Castle had been picked up for a third season on March 30, 2010. The third season order was for 22 episodes.
Season 3 premiered on Monday September 20, 2010.
Castle returns from a summer in the Hamptons with his ex-wife Gina to find he isn't exactly welcome at the precinct after unwittingly hurting Beckett. As he works to rebuild her colleagues' trust, he finds out that Beckett has a new man in her life, motorcycle-riding cardiac surgeon Josh Davidson. Despite being in separate relationships, their attraction remains as strong as ever. Meanwhile, a voice from Beckett's past blows her mother's unsolved murder case wide open and threatens the lives of everyone she loves, including her own, and tests her partnership with Castle.
- Beckett shows Castle her homemade murder board of her mother's case.
- A dead psychic's daughter predicts that Beckett will meet an "Alexander" who will become very important to her and "save her life" one day. She later discovers Castle's given name is Richard Alexander Rodgers.
- Martha's boyfriend Chet proposes to her, but before she can refuse him, he dies of a heart attack.
- Beckett is reunited with her old training partner and former flame, Mike Royce, but must arrest him.
- Castle and the guys at the precinct meet Beckett's new boyfriend, cardiac surgeon Josh Davidson.
- Castle and Beckett cross paths with the serial killer known as the Triple Killer, or 3XK, for the first time.
- Alexis begins dating her first serious boyfriend, Ashley.
- Castle buys the historic bar where he wrote his first novel, The Old Haunt.
- The actress set to play Nikki Heat in the upcoming movie version of Heat Wave, Natalie Rhodes stirs up trouble at the precinct when she shadows Beckett's team.
- Ryan proposes to his long-time girlfriend, Jenny, who accepts.
- Castle breaks up with his ex-wife, Gina.
- Lanie and Esposito begin a heated romance and fail to hide it from their friends.
- Beckett discovers her mother's murder was part of a larger, more sinister conspiracy and she arrests the man who ordered her mother's death. Meanwhile, Castle tells Martha his shadowing Beckett "isn't about the books anymore."
- Castle and Beckett share their first kiss.
- Martha inherits money from Chet's estate and uses it to open an acting school.
- Castle uncovers a terrible secret about an old friend from his youth who first encouraged him to express himself through writing.
- Castle and Beckett successfully defuse a nuclear bomb that could have destroyed New York City.
- Martha rekindles a relationship with an old soap opera co-star.
- Captain Montgomery is forced to put an old friend, the District Attorney, behind bars.
- Castle admits to being jealous when his protege seems to get too close to Beckett.
- Beckett and Castle travel to Los Angeles to investigate Mike Royce's death and Beckett comes close to avenging his death and cheating on Josh with Castle.
- Alexis decides to follow Ashley to Stanford for college and applies for early admission.
- The man who ordered Beckett's mother's murder escapes from prison and Beckett finds out not only that very powerful people are behind it, but that her own Captain and mentor had intimate knowledge of the conspiracy that he hid from her. In an effort to protect her, Captain Montgomery sacrifices himself, but at his funeral, Beckett is shot in the chest.
- As Beckett lies bleeding in Castle's arms, he tells her he loves her.