It's becoming increasingly common for studios to get working on sequels to films they think are going to be big before the previous film has actually been released. The hope is it will ensure they can shorten the amount of time between movies and that they'll also have a bit more time to get the script right.
They did it for Sherlock Holmes 2 and now they're doing it for the third movie in the franchise, with Deadline reporting they've hired Drew Pearce to script the period detective adventure movie. Robert Downey Jr. and Jude Law are likely to agree to complete at least a trilogy of the Holmes movies and with Sherlock Holmes: A Game Of Shadows expected to be a big hit with it opens in December, setting the third in motion is a bit of a no-brainer.
Drew Pearce is the creator of the British superhero TV satire No Heroics, which got him noticed in Hollywood (as least a US remake that didn't get picked up for him noticed), which resulted in gigs working on Iron Man 3, Marvel's Runaways and Pacific Rim.