katikat | Sherlock S3

Sherlock S3 starts filming on Monday, March 18!

Sue [Vertue] also kindly confirmed a second piece of information relating to filming on the new series. Long planned scheduling around the availability of both Benedict Cumberbatch and Martin Freeman means that principal photography on Sherlock Series Three will be done in two sections, with two episodes being filmed from March 18. A break will occur in the early summer, and then the third and final episode will be shot a few weeks later.

Sue commented: "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."

Source

Also, Paul McGuigan who directed 4 of Sherlock's 6 episodes won't be back for S3: link.