The latest run of the BBC thriller - which stars Idris Elba as maverick cop John Luther - recently began shooting.
Resident Evil actress Guillory will play Luther's new love interest Mary, who meets the rogue detective in a chance encounter.
"You realise why Luther is so good when you read Neil Cross's scripts, and then you meet Idris and understand what 'untamed' means," said the actress.
"There is no other set I'd rather be on, and I love the disastrous fun of being Mary Day, the right girl in the wrong place."
Four new 60-minute episodes of Luther will see Elba's character on a mission to bring down a ruthless ex-copper.
Idris Elba said: "I want to thank the fans for waiting so patiently for the new season. We're thrilled to deliver it to you. Here comes John!"
Warren Brown (Good Cop) will also return as Luther's partner DS Justin Ripley, alongside Nikki Amuka-Bird (DS Erin Gray), Dermot Crowley (DSU Martin Schenk) and Michael Smiley (Benny Deadhead).
Further guest stars include Lucian Msamati (Game of Thrones) and Ned Dennehy (Parade's End, Tyrannosaur).
Series creator Neil Cross has expressed his desire to produce both a Luther movie and a spinoff show featuring the character of Alice Morgan (Ruth Wilson) once the third series has aired.
> Idris Elba confirms Luther movie plans: 'It's a very strong goal'
> Luther creator 'plans to end show, make film'
Watch a BBC video confirming the third series of Luther below: