Jackson, who was responsible for the Oscar-winning adaption of Tolkien's "The Lord of the Rings" trilogy, said be began considering the possibility of three films after watching an early cut of the first Hobbit movie.
"We were really pleased with the way the story was coming together, in particular, the strength of the characters and the cast who have brought them to life," he said on his Facebook page.
"All of which gave rise to a simple question: do we take this chance to tell more of the tale? And the answer from our perspective as the filmmakers, and as fans, was an unreserved 'yes'."
The first film, "The Hobbit: An Unexpected Journey", will premiere in Jackson's hometown Wellington on November 28, with the second, "The Hobbit: There and Back Again", scheduled for release in late 2013.
Jackson did not reveal the name of the third movie or its release date.
The decision to add an extra movie follows a recent Hollywood trend of splitting a single book into multiple movies to maximise box office returns from blockbuster franchises.
The final novels in the Harry Potter and Twilight series have been stretched into two films and the same is set to happen with the last book in the Hunger Games saga.
Jackson said the decision to make three films was possible because of the extended appendices in the Lord of the Rings, in which Tolkien adds details of the Middle Earth fantasy world in which the Hobbit takes place.
"It has been an unexpected journey indeed, and in the words of professor Tolkien himself, 'a tale that grew in the telling'," he said.
Actors reprising their "Lord of the Rings" roles include Ian McKellen, who returns as Gandalf, Cate Blanchett as Galadriel, Orlando Bloom as Legolas, Christopher Lee as Saruman, Elijah Wood as Frodo, and Andy Serkis as Gollum.
British actor Martin Freeman, from "The Office", takes on the central role of Bilbo Baggins, who is swept into an epic quest to reclaim the lost Dwarf Kingdom of Erebor from the fearsome dragon Smaug.
Other big names appearing include Barry Humphries, Stephen Fry, James Nesbitt and Billy Connolly.