A "third Hobbit movie" certainly wouldn't be story from the book, "The Hobbit".
Even Jackson has referred to "the appendix material" when talking about this possibility.
It seems that he was intrigued about going back to the original idea of making a "bridge film" between the Hobbit and LOTR trilogy.
Essentially, I think this would be "The Tale of Aragorn and Arwen", plus a bunch of vignettes showing what everyone else was up to -- things we actually *do* know about from the appendixes, not invented stuff.
I'd actually really want to see an "Appendix movie"; it wouldn't be one narrative so much as several vignettes, but given how great the Extended Editions are....as they point out, when a movie is four hours long, you're not really worried about pacing anymore.