In the world of television, the concept of a writers' room is fairly common; by throwing a group of writers in a room, they can work together by breaking stories and helping to shape the overall arc of a series, a good fit for TV, but it's also becoming more common in the world of feature-films, with both the TRANSFORMERS and Universal Monsters franchise's setting up their own writers' rooms in order to bring their shared universes to life. It makes sense as those franchises all have multiple films lined up for the future, but in the case of SHERLOCK HOLMES 3, which is just one film, it's a little out of the ordinary.
Variety reports that Warner Bros. and Robert Downey Jr. have put together a writers' room to help shape the script and story for the next SHERLOCK HOLMES sequel. Included are Nicole Perlman (GUARDIANS OF THE GALAXY), Justin Malen (BAYWATCH), Gary Whitta (ROGUE ONE: A STAR WARS STORY), Geneva Dworet-Robertson (TOMB RAIDER) and Kieran Fitzgerald (SNOWDEN). Variety notes that it's rare for a room to be set up for just one film, but their sources tell them that in the case of SHERLOCK HOLMES 3 it "makes sense to gather a good group, rather than have several writers rewrite or polish, which happens to films often in the development process." It's been five years since SHERLOCK HOLMES: A GAME OF SHADOWS was released and Warner Bros. has been trying to get a sequel off the ground ever since. With this new group of writers working to hammer out the script, it seems that SHERLOCK HOLMES 3 is finally back on track.
As for when the sequel may shoot, it likely wouldn't be until late next year at the earliest as Robert Downey Jr. will be starting work on AVENGERS: INFINITY WAR in February 2017.