why so weak?

I get the feeling everyone was ready to move on to different projects and just wanted to get this one over with the close out the original trilogy.

I don't think it was significantly weaker, but yes like Sidha108 said, it seemed like it was somewhat rushed for other projects, and to me it seemed like a pilot/prototype more for the 'Scary Movie' franchise. 5/10.

Because the script was written by a different guy who did not even get the first two movies.

It's the weakest of the original three, but I still like it and I prefer it to 4. The series was originally supposed to end on 3, so maybe it was that people were already lining up new things and just wanted to get this out of the way. None of which excuses Courtney Cox's hair.