According to reviews, the new Pirates of the Caribbean 3 film is an absolute turkey. On top of early reviews from American suggesting that Shrek 3 is also a miss rather than a hit and the widely accepted view that Spiderman 3 is also the weakest of the three films in that series (although I liked it best), there seems to be something of a trend developing in serial films which are not based on books.
Films based on books (Lord of the Rings, James Bond, Harry Potter) or scripts written many years before (Return of the Jedi) don't seem to suffer from second sequal syndrome. Why is this ? Perhaps it is like third term government (like we have now) in that they lose direction.
Historically there have also been some duds from third films (Batman), but also some which have improved the franchise (American Pie : The Wedding - Back to the Future 3).
I am just hoping that the new film in the "Bourne" series, out in August will not suffer the fate of Shrek and Pirates. Then again, it is based on a book so it should be great.