The first episode of Sherlock series 3 was seen by 3.5 million people on iPlayer

"A quarter of television viewers are now watching their favourite dramas on catch-up services, it has emerged, as Sherlock becomes the most-watched programme after initial broadcast ever.
The television show gained an extra 3.5 million viewers from recordings and on the BBC’s iPlayer, beating Downton Abbey to become the highest catch-up audience in history.
The New Years’ Day episode, which revealed how Sherlock faked his own death, has now received a total of 13 million viewers, once the newly announced consolidated figures have been taken into account.
The record had previously been held by the 2011 Downton Abbey Christmas special, which saw character Matthew Crawley propose to Lady Mary and received 3.15million views after its initial broadcast on ITV.
A breakdown of the top ten shows to be watched by audiences from recordings or on digital catch-up services suggests around a quarter of viewers now prefer to take advantage of the service."

Source:  The Telegraph, 10th January 2014