The Steven Moffat-penned episode will be titled 'His Last Vow', which is a reference to Arthur Conan Doyle's short story 'His Last Bow' (also the name of the collection in which the story appeared).
The first and second episodes are titled 'The Empty Hearse' (The Empty House) and 'The Sign of Three' (The Sign of Four) respectively.
'His Last Vow' will be directed by Doctor Who's Nick Hurran.
Filming on the episode is due to commence in two weeks' time, with the first two episodes having wrapped in May.
> 'Sherlock' at Comic-Con: 10 things we learned
> 'Sherlock' at Comic-Con: panel live blog
Watch Steven Moffat and Mark Gatiss discussing the third series of Sherlock with Digital Spy at Comic-Con below: