Gatiss - who also plays Mycroft in the BBC drama - appeared to confirm that the first episode of the new run will be an adaptation of Sir Arthur Conan Doyle's 'The Adventure of the Empty House'.
The Sherlock series two finale - which saw Holmes (Benedict Cumberbatch) apparently plummet to his death - was a loose adaptation of Conan Doyle's 'The Final Problem', with 'The Empty House' being the next story chronologically in the canon.
"There's certain things about 'The Adventure of the Empty House' which feel set in stone because that's how Sherlock comes back, but at the same time we feel free to invent and to introduce new stuff to it," Gatiss told PA.
"I always found it a little unlikely that Dr Watson's only reaction was to faint for instance - as opposed to possibly a stream of terrible swear words."
The writer and actor added that the "overall feel" of Sherlock's third series will dictate which Conan Doyle tales will make up the second and third episodes.
"Steven [Moffat] and I have our all-time favourites, but it's really a question of what will fit into the structure," he explained.
The third series of Sherlock is expected to begin filming in early 2013.