Judging from the IMDB, the chicken isn't making golden eggs any more... I guess the character are getting tired and don't have that much to tell... here's what the NY Times has to say:
For all I know, there may be an endless supply of “Shrek” sequels in the pipeline. That DreamWorks ogre’s skin is the color of money after all. But there is nonetheless a feeling of finality about “Shrek the Third”, a sense that the tale has at last reached a state of completion. it goes on about the character reaching maturity and entering fatherhood, and so on...
Still, the popular vote is clear, with imdb scores decreasing from 8 (Shrek 100K+ votes) to 7.7 (Shrek II - 50k+) to 6.9 (3k+). Even accounting for the fact that early viewers may be hard core fans, and therefore harsher judges... the drop is severe and I don't think it will be recovered. Quite simply, the franchise has lost its allure. Even the fact that less people bothered to vote for the second is witness to this.
Well, as usual I doubt I will have time to go and see it in the theater, hence I will wait for a decent dvd rip to pop up on the net (ever heard of aXXo?) - and enjoy one of those boring fall afternoons.