Peter Jackson announced that what was originally going to be two Hobbit movies would be expanded into a full-fledged trilogy because he had so much footage. As of this writing, I haven't heard anything about how he plans to organize the material and structure his three movies. But looking at all the material he wants to or might include in his movies, this is how I would structure the three films.
The first movie focuses largely on Bilbo. He is in all but one or two scenes. Movie one also lays the groundwork for the White Council storyline. The climax is the fight with the spiders in Mirkwood and ends with Bilbo and the dwarves getting captured by the elves.
The bulk of the second movie covers the White Council storyline. Gandalf and company expel the Necromancer from Mirkwood. This is the big climax of the second film. Meanwhile, Bilbo breaks the dwarves out of prison and they escape down the river in barrels. There might have to be some contrived set pieces added to this sequence to flesh it out. Plenty of time is spent in Lake-town, convincing the residents of Lake-town to help our band of adventurers. This gives plenty of opportunity to introduce Bard the Bowman as a major character and offers ample screentime to Stephen Fry as the Master of Lake-town. It ends with Bilbo and the dwarves heading off the the Lonely Mountain.
The third movie focuses on two major set pieces: Bilbo's conversation with Smaug and the Battle of Five Armies. This allows for plenty of screentime to be devoted to the politics that lead up to the battle and Thorin's case of dragon fever. The movie opens with a flashback to the battle of Azanulbizar. The movie ends with Bilbo saying, "Thank goodness."
Possible titles for the three movies: The Fellowship of the Burglar, The Two Fortresses, and The Return of the King Under the Mountain.
The big problem I see with this structure is that movie two has very little of Bilbo. However, the book states that Gandalf is "finishing up" his business with the Necromancer as Bilbo and the dwarves are floating down the river in and on barrels, so the chronology works pretty well.
Of course, if I was in charge, there wouldn't even be three movies. I would try to do the whole thing in one movie, sticking to the material in the book and ignoring the material in the Appendices and other works. Hopefully I could get the run time down to about two hours and get a PG rating. After all, it's a children's book, which means it should be a children's movie.