Yes there will be three movies.
This is made possible by expanding on the unexplained details from the Hobbit that are explained elsewhere (LoTR appendices, Simarillion, Unfinished Tales ...).
For instance, the "white council" from the first movie was not in the Hobbit. In the Hobbit (book), Gandalf just disappears for a bit and comes back without much explanation. But it is explained in the LoTR appendices and was adapted for the movie.
Originally it was going to be 2 movies, but Peter Jackson found that he had so much material he could cover that he might as well make three.
