http://www.theonering.net/torwp/2012/08/02/60244-the-bold-hobbit-trilogy-decision-and-what-to-expect-now/
Great article that does a good job of explaining why the decision for a third film was not based on money. Like the article says, I think it was a simple matter of having too much footage for two films and an unwillingness to cut stuff out. And that opened up a golden opportunity to include even more material from the appendices.
I don't think the decision will have a huge effect on film one. If anything, the cutoff point might be moved up to the company's capture in Mirkwood. The big plus here is instead of having an overly long and bloated film two, you get to not only keep everything, but fill in even more things during and after the events of The Hobbit, and split that over two films.
My educated guess would be that film two ends after Smaug attacks Laketown, and the White Council and Dol Goldur stuff taking place in that film. Then film three will be totally centered on the Battle of Five Armies, the aftermath, and an extended prologue with the years leading up to Fellowship.
The biggest change, in all likelihood, is that we'll get three 2+ hour films instead of two 3+ hour films.