Steven Moffat and Mark Gatiss attended the MediaGuardian's Edinburgh International Television Festival panel to discuss Sherlock. They were joined by actor Andrew Scott (Moriarty) and producer Sue Vertue, to answer various questions. The big news was the announcement that series 3 will film between January and April 2013, suggesting a late-summer/early-autumn premiere next year.
They also teased the next three 90-minute specials, with the one-word clues "rat", "wedding" and "bow". Fans will remember that series 2's clues were "woman", "hound" and "fall"—and much easier to predict contents as a result! That said, "wedding" suggests Watson (Martin Freeman) will finally be getting married to his fiancé, to the likely chagrin of Sherlock (Benedict Cumberbatch).
It's already been revealed that the "rat" premiere will be partly inspired by Arthur Conan Doyle's story "The Adventures of the Empty House", where Holmes made his miraculous return from the dead. It also seems likely the premiere will be taking a cue from "The Adventure of the Sussex Vampire", where Holmes famously refers to the case of the Giant Rat of Sumatra, describing it as "a story for which the world is not yet prepared". My guess is that Moffat/Gatiss are going to tell that story, because the line has inspired numerous Sherlock-related fictions over the years. As for "bow", this might refer to "His Last Bow", which was a story set on the eve of World War I where Sherlock posed as a British informant called Altamont to German agents. Will this story be updated to reflect event in the Middle East somehow?