I think that the movie hints at the direction that the manga is taking, such that it has a context in the story. I guess it's logical to hypothesize that there will be, at the very least, a time skip involved, considering that everyone has grown up. I don't know whether he could drag the plot for an entire part, however, considering that it seems that many major plot points and themes are already well on their way to reaching a resolution.
Right. I get all that. But there is no evidence to suggest that the time skip happens within the manga. It seems more likely that the manga will conclude as Kishimoto has said, and the movie will function more as an epilogue -- as the movie's title, "The Last", suggests.
I wouldn't consider the story continuing in film as a Part 3. There would only be a Part 3 if the manga continued. Otherwise it's just Part I, Shippuden, and a film epilogue.