I am a huge huge The Hobbit fan, and a decent fan of TLOTR trilogy. I am actually happy to have heard about the fact that it will be 3 films instead of two. Heres why.
The first film will be a decent chunk of the book. It will not feature Smog or anything after Smog. It will likely be very true to the books. This includes the bit at Bag End, a bit with the Trolls, the part where they are at Rivendell (likely extended because they seem to want to include a female character not found in the book & who is an elf), then the large bit with the goblins and the famous scene with Golum and the ring. I bet it will end with them starting to enter Mirkwood.
The second film will feature a lot more of Smog and will lead UP TO the Battle of the Five Armies. It will feature an extended part about Mirkwood. More elves. The humans. Smog. And ending with Smogs death.
The final film will have the final battle. Obviously this bit will have a huge extended portion away from the books.
Also extended from what was in the books: They will go very much into detail about where exactly Gandalf was when he disappeared from the group all those times. This really will tie into the LOTR trilogy more, where Sauron came from, perhaps about the ring, etc.
Also, Beorn.