A Glock 26 is carried by Beckett as her backup. This pistol is seen used by Castle in order to defend himself in the Season 3 premiere "A Deadly Affair" (S3E01).
In "Under the Gun" (S3E03), Gayle Carver (Sophina Brown) is seen wielding this gun during a confrontation between the detectives Mike Royce (Jason Beghe) and Clifford Stuckey (James Handy).
Montgomery uses a Bond Arms Snake Slayer IV Derringer as his holdout weapon during his confrontation with Lockwood in Season 3's finale "Knockout" (S3E24). It is the IV version of the Snake Slayer as noted by its longer barrel, and although normally chambered in .45 this pistol appears to use the .38/.357 barrel, as Montgomery loads it with the same rimmed rounds he uses in his revolver.
In "Punked" (S3E04), Daniel Goldstein and Adam Murphy are found to have each used flintlock pistols in a duel to impress a woman. Later, while trying to determine whether to charge Adam with murder or manslaughter, they hilariously succeed at hitting everything in the range but the targets. After several failed attempts, Castle uses duct tape to attach a scope, and laser sight in a last ditch attempt to hit the target. Other models are also seen being held by Ivan Podofski (Muse Watson) in photos on a antique firearms social networking page.
A Smith & Wesson Model 19 is seen loaded by Captain Roy Montgomery (Ruben Santiago-Hudson) in Season 3's finale "Knockout" (S3E24). He later seen with in the hangar in the episode.
In "Under the Gun" (S3E03), bail bondsman Deon Carver is seen brandishing his Model 60 when he returns to his office and notices somebody has broken in. Castle is seen holding a snubnose revolver as Beckett, Ryan & Esposito clear a murder victim's apartment.
In "Punked" (S3E04), Castle is seen wielding what appears to be a custom engraved replica of a Colt 1851 Navy revolver while bursting through his front door and catching his daughter snogging her boyfriend.
Esposito (Jon Huertas) uses a Remington 870 on multiple occasions throughout the season. In "Sucker Punch" (S3E13), the victim Jack Coonan (Justin Alvarez) had one loaded with hollow point slugs in his apartment.
In "Under the Gun" (S3E03), Mike Royce (Jason Beghe) is seen wielding what appears to be a Mossberg 500AT shotgun during a confrontation between the detectives, Gayle Carver (Sophina Brown) and Clifford Stuckey (James Handy).
In "Knockdown" (S3E13), Hal Lockwood (Max Martini) uses a suppressed ROCK SOPMOD M14 to shoot at John Raglan, and later during a shootout with Beckett. It appears to have a Harris bipod and AN/PEQ-2/5. It is supposedly chambered in .338, despite the round being much too large for fit the 7.62x51mm chamber.
In "The Final Nail" (S3E15), detectives arrive at the crime scene to find a female victim with multiple flat-head nails protruding from her skull. Behind a nearby chair they find a nail gun. It appears to be a cosmetically altered KitPro series judging by the shape of the butt, housing assembly (screws pointing aft), housing vents on the side and top, and the 'over/under' 'muzzle'. I think the 'over' is a nail depth gauge.
