The special will air over the festive period followed by the six-part series in 2017.
The duo will reprise their roles as Vicar Sidney Chambers and Police Inspector Geordie Keating, James Runcie’s unlikely 1950s crime fighting duo.
Also back is Tessa Peake-Jones as Sidney’s sharp tongued housekeeper Mrs Maguire who keeps the vicar in check, Morven Christie as Sidney’s lover Amanda, Al Weaver as curate Leonard Finch, Kacey Ainsworth as Geordie’s long suffering wife Cathy and Lorne MacFadyen as DC Phil Wilkinson.
The Christmas special, the show’s first, will see Amanda adjusting to life having left her husband, Guy (Tom Austen). The love between Sidney and Amanda is palpable, but can a vicar in the 1950s really be romantically involved with a married, pregnant woman?
ITV says viewers “can expect a festive, snow-filled Christmas episode full of emotion and drama.”
In third series new Archdeacon Gabriel Atubo reminds Sidney that as clergyman he must put duty above his own needs and lead by example. Given the strength of his feelings for Amanda, is this something that Sidney can do?
On the face of it, Geordie is still happily married to Cathy. But there continues to be an undeniable spark between himself and feisty police secretary Margaret (Seline Hizli).
Leonard was left heartbroken after Daniel (Oliver Dimsdale) left him for another man, but in this series he appears to put this behind him and find romantic happiness with someone else. But will those around him allow him this happiness?
Mrs Maguire’s friendship with dashing Jack (Nick Brimble) develops, but can anyone ever replace her late husband Ronnie?
The heart of the show remains the friendship between Sidney and Geordie. As they solve crimes in Grantchester, Cambridge and beyond, they each deal with personal crises at home, once again their unique and unlikely friendship is put to the test.
Peter Davison will guest star in the series as Geoff Towler, the village solicitor, captain of the cricket team and a man who rules his family with a rod of iron.
Grantchester is produced by Kudos, and will be executive produced by Kudos’ chief executive Diederick Santer (Cutting It, EastEnders) and series produced by Emma Kingsman-Lloyd (DCI Banks, Lip Service).
Santer said: “Grantchester continues to be a delight to make and I hope will continue to delight the viewer.
“To be bringing to the audience a third series AND a very special Christmas episode is a particular thrill, and we can’t wait to be pouring snow onto the East Anglian Fens.”
“Viewers have really taken Grantchester to their hearts,” said Victoria Fea, ITV’s Controller of Drama.
“We’re really excited to commission not only a third series, but also a Christmas special. The village of Grantchester is always beautiful, but to add a festive touch will be absolutely wonderful!”