Originally The Hobbit was planned as two films. Rumors have been flying around that Peter Jackson would like to make it into a trilogy. When I first heard the rumor I had little doubt that this wouldn’t happen. There is no reason that the studio wouldn’t want to add another film that is almost guaranteed to make big bucks. I understand it’s hard for a studio to argue with more money.
My take is if the story flows and there is enough for three movies, go for it, make three movies. However, if things drag and much of what is made into a third film should have been left on the cutting room floor or saved for a blu-ray release it’s not worth it.