I found the third one to be mildly enjoyable. But I also found it to be a cut off point. Movie trillogies are always weakest at the third film. Back to the Future, Star Wars (original saga 4-6), Ninja Turtles... the third in each one is least liked by fans (though a turn around would be the new Star Wars trilogy- where the third one was the best of the bunch).
Franchise films are hit or miss. James Bond is a strong franchise, but had some terribly weak films in the series. The last Pierce Brosnan one had no fans, be they critics or movie goers. And a lot of Land Before Time fans HATE the DTV sequals. Can't say I don't agree with them. Yuck.
I feel in Shrek 3 they exhausted everything they've been doing:
- Shrek is an ogre, and therefore disgusting. be it his food choices, or his hygine
- Donkey and Pusspuss can't get along, but are still friends reguardless
- The Princess is proud to be what she is, but wants to be normal
- Kattsenburg wants to stick it to Disney over and over again 1
- Random pop songs in liu of real, written, original pieces 2
- Shrek gets angry at everyone, feels sorry for himself, and explains that he's different and can't handle something 3
Footnotes:
1- Every Shrek film has on some level made a rant about Disney's films. I think the Princess resistance force was clever, though.
2- The King's funeral procession using "Live and Let Die"- a James Bond song that is actually about killing people- pretty much shows they're trying a little too hard to shove that sort of thing in.
3- Has also been seen in the Christmas Special, as Shrek doesn't know how to handle a holiday he never celebrated due to being alone.
There's little room for growth. They're running out of Fairytales to satirize (and for the record, Peter pan shouldn't really be there, and King Arthur is a legend, not a Fairy tale). I'd love for them to do more obscure, foreign ones (like Momotaro- the Peach Boy from Japan) as that's pretty much the only thing left.
I liked how Shrek was this big Outside the mainstream style film, sticking it to Disney when it was first released... but now it's so mainstream that someone really needs to stick it to dreamworks.