The BBC said it was its "most-watched drama series in over a decade". The "consolidated" figures include not only those who watched it live but also people who recorded it or used catch-up services over the following seven days from first broadcast.
Viewers saw the last episode climax with what appeared to be the possible return of arch-villain Moriarty.
Figures have already shown that the first episode of the series - starring Benedict Cumberbatch as Holmes and Martin Freeman as sidekick John Watson - had the biggest-ever catch-up audience, with 3.5 million watching since the first screening.
It had a consolidated audience of 12.72 million, while the other two programmes had 11.38 million.
BBC1 controller Charlotte Moore said: "This latest accolade is the icing on the cake and only further demonstrates the audience's huge appetite and appreciation for original British drama on BBC1."