Why three Hobbit movies? Here's my guess.
In thinking about The Hobbit today, I've hit a guess as to the proximate reason that Jackson decided he wanted three films. (The studio's reason is clearly "more money".) I think it may be significant that nobody has ever said a word about where Jackson planned to split the story into two films. My guess is that after watching a rough cut of most of what he's filmed, Jackson couldn't find any good place to make that split... but with a bit more work, he he could see a good way to split it in three.
My guess is that film 1 will run from the start of The Hobbit to the final escape from the Misty Mountains: an intense sequence fighting with goblins and wargs and a rescue by the eagles. Film 2 will be a lot like Jackson's The Two Towers: those on the primary quest will travel slowly toward their mountain destination until everyone but a single hobbit is locked up, while a secondary thread will split off and wind up occupying most of the movie's runtime with a single battle scene blown wildly out of proportion beyond the attention Tolkien actually gave it (the White Council driving Sauron from Dol Guldur, in this case). Film 3 will center on the escape to Lake Town and the various confrontations with Smaug, and will wrap up with the (overemphasized) Battle of Five Armies (and "too many endings" again). (All three films will pull in substantial chunks of historical or auxiliary content from the LotR appendices.)
My guess is that film 1 will run from the start of The Hobbit to the final escape from the Misty Mountains: an intense sequence fighting with goblins and wargs and a rescue by the eagles. Film 2 will be a lot like Jackson's The Two Towers: those on the primary quest will travel slowly toward their mountain destination until everyone but a single hobbit is locked up, while a secondary thread will split off and wind up occupying most of the movie's runtime with a single battle scene blown wildly out of proportion beyond the attention Tolkien actually gave it (the White Council driving Sauron from Dol Guldur, in this case). Film 3 will center on the escape to Lake Town and the various confrontations with Smaug, and will wrap up with the (overemphasized) Battle of Five Armies (and "too many endings" again). (All three films will pull in substantial chunks of historical or auxiliary content from the LotR appendices.)