Trilogies

I have a theory (as of 2 minutes ago) why it seems trilogies of movies are so common. This does not apply to movies based on books that are already trilogies (Lord of the Rings), but I guess it can apply to books as well, but only the ones where the story lines are basically independent and only share the same characters, etc.

My theory is thus: If Movie I is a success, Movie II will be on the cards to make more money out of the franchise. In Movie II it is important for the producers to try new things in order to differentiate from Movie I. However, Movie I was a successful movie, and changing too much will make Movie II look crap in comparison. But not to worry, many changes are made, and Movie II is released and possibly thought to be crap my many viewers. Along comes Movie III. Movie III does not have to live up to only the expectations of Movie I, but also Movie II, which are quite low. So all the producer needs to do for Movie III is reproduce what they did for Movie I, and the fans will love it.

I haven't seen Ocean's 13, but from what I hear, this theory holds. Can anyone think of other movie franchises that follow this rule? Of course, there are always exceptions, some movies just continue to get worse as more installments are added, eg, The Godfather, and until recently, Batman. I'm not sure how to classify the Matrix trilogy. It was always intended as a trilogy, but Movie I was excellent, Movie II was shit, and Movie III improved but did not quite live up to Movie I.

I have an urge to watch the Matrix movies again, I must have seen the first one half a dozen times, but the other two only once.