Deputy US Marshal Raylan Givens (Timothy Olyphant) carries a Glock 21 as his sidearm for the majority of the series.
Glock 17
Nearly all of the other US Marshals in the series carry full size Glocks as their sidearms as well including Chief Deputy Art Mullen (Nick Searcy) and Deputy Marshal Tim Gutterson (Jacob Pitts).
Glock 19
In "Cut Ties" (S3E02), what appears to be a Glock 19 is seen as the sidearm of Assistant Director Karen Goodall (Carla Gugino).
Glock 26
The Glock 26 appears in the series mainly as the sidearm carried by Deputy Marshal Rachel Brooks (Erica Tazel). The Glock 26 also is the pistol Raylan keeps as his backup as well. Throughout the series, he is seen carrying it in the back of his waistband.
In "The Gunfighter" (S3E01), Fletcher 'The Ice Pick' Nix (Desmond Harrington) is carrying a nickel M1911A1 that he uses to play a drawdown game with his victims. Raylan is also seen with the pistol.
Walther PPK
In "Cut Ties" (S3E02), Terry Powe (Frank John Hughes) is seen using a Walther PPK. Art Mullen is also seen holding the weapon.
Rifles
Remington 700PSS
In "The Gunfighter" (S3E01), Gutterson is seen providing cover with a Remington 700PSS.
Shotguns
Mossberg 500AT
The Mossberg 500AT shotgun is a frequently seen weapon in Season 3.