The Guy Ritchie-directed Sherlock Holmes movie franchise - headlined by Robert Downey Jr. as the eponymous master mystery-solver - has been inactive since Sherlock Holmes: A Game of Shadows was released back in late 2011. Warner Bros. Pictures actually started development on Sherlock Holmes 3 before A Game of Shadows had even opened in theaters, but the film still wound up on the back-burner due in no small part to RDJ's commitment to the Marvel Cinematic Universe (which has kept him largely preoccupied for the past five years or so now).
Downey Jr., during the press tour for Captain America: Civil War, revealed that he, Ritchie, and costar Jude Law (who plays the Dr. Watson to RDJ's Holmes) are working towards getting filming on Sherlock Holmes 3 started this year. It remains to be seen if that's plausible given RDJ's other commitments - namely, his role in Marvel Studios' Spider-Man: Homecoming and the upcoming Avengers: Infinity War films - but Sherlock Holmes 3 now has a new writer onboard, to help get the movie off the ground at last.
The initial Sherlock Holmes 3 script was penned by Iron Man 3 and Mission: Impossible - Rogue Nation co-writer Drew Pearce. Deadline is reporting that the screenplay is now getting a rewrite from James Coyne: a writer who, in 2015, put together a fresh script draft for Treasure Island, the re-telling/re-adaptation of the Robert Louis Stevenson swashbuckling adventure novel that Ritchie was once attached to direct (and might yet direct sometime in the future, for all we know). Treasure Island is still being produced by Ritchie's writing/producing partner, Lionel Wigram, who's likewise working on Sherlock Holmes 3 too.
It remains to be seen which (if any) of Sherlock Holmes creator/author Sir Arthur Conan Doyle's many stories about the brilliant sleuth will provide inspiration for the storyline in Sherlock Holmes 3. Ritchie's first Sherlock Holmes installment (released back in 2009) contains nods to Doyle's Holmes stories like the novel The Hounds of the Baskervilles, while A Game of Shadows is partially based on Doyle's famous Holmes short story, "The Final Problem". Sherlock co-creators Mark Gatiss and Steven Moffat also adapted 'The Final Problem" with the show's second season with the episode "The Reichenbach Fall". The Sherlock show-runners have since adapted Holmes stories such as "The Adventure of Charles Augustus Milverton" to the small screen, while at the same time referencing Doyle's collection of Holmes short stories, His Last Bow, with episode titles like "His Last Vow".
Sherlock Holmes 3 could follow Sherlock's lead and pit RDJ's Mr. Holmes against a new big screen version of the Charles Augustus Milverton character - a blackmailer who, in Doyle's literature, Holmes famously detested more than any other killer or criminal he had encountered in his many cases. Lars Mikkelsen played the villain (renamed Charles Augustus Magnussen) in "His Last Vow", but Ritchie, Coyne, and their collaborators may opt to instead draw from one of Doyle's many other Sherlock Homes antagonists and/or stories that have yet to be adapted for film and/or television. There's certainly no shortage of source material that can be used here.
As to the likelihood that Sherlock Holmes 3 will enter production this year: that's up in the air. Ritchie is currently in post-production on his King Arthur film (costarring Law). Meanwhile, RDJ is attached to a slow-developing Pinocchio adaptation that Ron Howard is currently set to direct - in addition to having more MCU films on his plate. Scheduling will thus be a difficult obstacle to maneuver around in this case; all the same, there's still clearly a demand for one final trip through Ritchie's version of Victorian England (fisticuffs, slow-motion gun battles and all) with RDJ and Law's Holmes/Watson.
We'll bring you more information on Sherlock Holmes 3 as it becomes available.