I don't think it'll end on a straight lead-up to LOTR. This is The Hobbit. At the end of the day it's the story of Bilbo Baggins. The events of the movie take place sixty years before FOTR even begins. In the first trailer (before the duology became a trilogy) we saw Gandalf exploring a dark fortress (likely Dol Guldur) and getting into a fight with someone (not going to mention his name to avoid spoilers), so I think that'll be in DoS.
There's only fifty pages between the death of Smaug and the end of the book, but there's a lot of material in there to cover. If DoS doesn't include all of the battle at Dol Guldur then there will be that to wrap up, and there will also be the aftermath of Smaug's attack on Lake-town. Bilbo has to discover the Arkenstone, then the Wood-elves and the Men of Lake-town claim Erebor's treasure and put it under siege, Bilbo sneaks out to speak to Bard, Gandalf returns, then Dáin shows up with his army of Dwarves and the Orcs show up, and the Battle of Five Armies begins. Then there's the aftermath of the battle, and Bilbo's journey back to the Shire through Beorn's home and Rivendell, and then his having to deal with what happened to Bag End while he was gone. That's more than enough for the third movie to cover without having to bother with any unnecessary LOTR tie-ins.
Besides, I think if they're going to do anything with Aragorn in his pre-LOTR days, it would be in its own movie (there actually is enough material in the Appendices for that, with some expansion for dialogue and such of course).