Steven Moffat confirms writing is underway on Sherlock Series Three

During an appearance on BBC Radio 5 Live this afternoon, Steven Moffat has confirmed that Mark Gatiss and Steve Thompson are hard at work writing their respective episodes of Sherlock Series Three, though due to his commitments on Doctor Who he has yet to begin work on his installment.
Aside from Mark’s confirmation that he is writing the first episode of the series, there’s still no word on which episodes of the series Steven and Steve are writing.
Steven reconfirmed that filming on Sherlock Series Three will begin in January 2013.

source: sherlockology