OK, I had already decided on that title before I saw the film, but it's appropriate.
There's something amiss in SHREK 3 from frame 1. The humor isn't quite right, the story doesn't flow, and the characters aren't on target.
The writers are new. And it shows. The characters just don't work the way they did in the first two films. The first film was pretty good at wrapping it up in the end, and the second even more so, so in the third installment, a lot is done to undo what was done in the first two, then redo it so they end up in the same place. Which is not entertaining to watch.
Puss 'n Boots is the newest of the old characters and manages to stay somewhat fresh until he is forced to trade places with Donkey. Most of the new characters in this film are one-dimensional and exist only as cliches or cheap jokes. There are a few moments of humor that rise above sitcom moments (Gingerbread Man's life flashback) but most of the film is terribly written. About the best thing I can say about the movie is that the third act is the best part of the film, but the climax is still weak and confusing when Shrek beats Prince Charming yet again. At least Eric Idle's Merlin is humorous.