But she added that it was still too early to say when it would be transmitted.
Series 2 of the BBC One drama aired in January this year and ended with Sherlock Holmes, played by Benedict Cumberbatch, faking his own death after a rooftop showdown with Jim Moriarty (Andrew Scott).
In the meantime, Cumberbatch (the son of Wanda Ventham) is filming for the next Star Trek movie, set to be released in May 2013, and The Hobbit, the latter of which stars Martin Freeman - Sherlock's sidekick Dr John Watson - in the lead role of Bilbo Baggins and is scheduled to be released this December.
Sherlock was co-created by Vertue's son-in-law Steven Moffat and Mark Gatiss.
newslink: Radio Times