I have read in several places that the movie scene differs quite a bit from the book scene. Sadly I haven't read the books. I just want some major highlights as to how the 2 scenes differed, and what impact the differences could have in the third film.
Well, there's a site with the book's scene of Bilbo and Smaug's meeting here (turn your speakers off because there's some annoying MIDI music that plays automatically), so you can read it and compare it to the movie version. In the movie Smaug initially seems to sense Bilbo's ring and Bilbo takes it off out of fear, becoming visible, whereas in the book Smaug doesn't say anything about the ring and Bilbo remains invisible, but after that their initial conversation with Bilbo's riddling descriptions of himself is basically an abridged version of the dialogue in the book, taking a lot of specific lines from it, as you can see here:
The video below has a lot of overlap with the one above but at 2:04 it shows the dialogue immediately after the end of the previous video, showing that Smaug seems quite certain the dwarves have come back to reclaim their treasure, whereas in the book Smaug seems to have some idea Bilbo has been around dwarves but mainly suspects the lake-men:
Then in the movie scene below Smaug talks about how Bilbo is being used by Thorin Oakenshield, whereas in the book Smaug doesn't mention the name of Thorin Oakenshield (and note that the first answer to this question suggests this is a mistake, since although Smaug could have been familiar with Thorin, Thorin didn't get the name "Oakenshield" until after Smaug had already taken the treasure). Also in this scene Bilbo just accidentally glimpses the chink in Smaug's armor, whereas in the book he had used leading questions to trick Smaug to show off his armored belly. And in the movie Smaug knows that Thorin wants the arkenstone and says how he's tempted to let him get it since it will "corrupt his heart and drive him mad", but in the book Smaug doesn't say anything about the arkenstone.
As for "what impact the differences could have in the third film", that involves some spoilers so anyone who doesn't know the plot of the Hobbit and would rather be surprised should stop here, but basically these changes are all just giving Smaug knowledge of plot points that were in the books but that he didn't know about there (perhaps so he can foreshadow them), like the power of the ring, and the identity and motives of Thorin (who will later have a falling-out with Bilbo when Smaug is destroyed, because of his greed for the treasure and especially the arkenstone). There's nothing in the changes that suggests ways that Jackson is likely to deviate from the book in the third movie.