The date was confirmed to website Sherlockology by the show's producer Sue Vertue.
Principal photography is having to be done in two waves, though, because of the availability of stars Benedict Cumberbatch (Holmes) and Martin Freeman (Watson) for filming. Two of the three episodes will be shot at first, with cast and crew then taking an early break for the summer before returning a few weeks later for the final episode of the series.
Vertue, who is married to the show's co-creator Steven Moffat, said:
We could never have fitted all three films into one long go because of everyone's availability. It's being flexible like this which allows us to keep making them.The start date coincides with the beginning of filming on the 50th-anniversary episode of Doctor Who - and Moffat is also the showrunner of the sci-fi series.
Last Thursday, Freeman told Graham Norton, on the latter's Comic Relief's Big Chat on BBC Three, that even after reading Mark Gatiss's "brilliant" script, he and the rest of the cast still weren't entirely sure how Holmes had managed to survive his rooftop fall at the end of Series 2. Gatiss is the fellow co-creator of the show.