Spoiler heavy. You have been warned.
What was always going to make or break this adaptation for me was the planned addition of story elements not shown in the book, though they were story elements that existed at the time of the story's events. By that I'm referring to the stuff involving the Necromancer of Mirkwood.
And it came off clumsy. The reason is that Jackson, for whatever reason, is rewriting that story entirely. At the time that Bilbo and the Dwarfs set out to Erebor, the Necromancer of Mirkwood was already a known entity and the White Council had already decided to move against him. Gandalf was on this journey with Thorin and Company specifically for this reason. When Gandalf repeatedly disappears, he's off handling business related to this incursion. He then leaves everybody when they get to Mirkwood to go off and handle driving the Necromancer (actually Sauron) out.
So, the forcing... and I use this term intentionally... of the Necromancer story elements into the story of the Hobbit just felt off. It makes Gandalf's reason for being with Thorin sketchy in the first place, they had to add things like Radagast the Brown (gah...) informing Gandalf of things going on in Mirkwood, and they added in a rather stupid element where the White Council might have opposed and somehow stopped Thorin from continuing his quest. Of these, Radagast's portrayal is just obnoxious and childish, but the main issue is the last one. NOBODY cared that Thorin was going to try to retake Erebor and certainly NOBODY was going to try to stop him. There was never any concern that Smaug would join Sauron, and even if there was, having Thorin go and kill the beast would have been a good thing. So, none of it made sense. Having the dwarfs have to somewhat ESCAPE Rivendell made NO SENSE AT ALL.
I have a few other minor beefs, but... they're minor, so they're not important. The famous last words of the Goblin King, for example, made me (and most of the audience) groan. Just a really stupid idea. But the big issue was the added stuff. Contrarily, I didn't care much for the severely increased importance of Azog the Defiler (real character, but in the book he WAS killed at Moria by Thorin), and by that I mean I don't mind it. Gives Thorin a good dramatic reason to fight and die in the Battle of Five Armies instead of just being mortally wounded somehow in the frackus as he was in the book.
I found this movie more humorous and entertaining than the first two. However, it is easily the weakest one story wise. They literally hand wave how the animals get to Monte Carlo in the first place, and from there on it's just one coincidence after another to get them to the circus, where most of the movie's relevant plot takes place. From there on, they don't bother going into great depth with each character and their respective take on the circus life. The movie focuses the most on Alex for some reason, with Marty, Gloria, and Melman relegated to near insignificance. As for said plot, meh; despite the visuals, especially of the circus acts, were stunning, it's just not great material overall.