Guidelines for a third installment?

I liked this movie. I thought it was funny, and the story had a lot of twists and turns. I liked the message at the end, even if it was somewhat reminiscent of "Mr. Deeds" (and whoever used it before Mr. Deeds). It sounds like the only problems people had with this movie are that it was a sequel, and that the first 2 movies were really good. The fact that it was a sequel probably helped it to make more money, so maybe there deserves to be a backlash, but some Part 3's seem to be accepted more than others.

I've comprised a list of criteria for the third and later installment of a series to be accepted by critics. This is what I have so far. Please feel free to make any comments or additions:

1. There must be at least 10 years since the previous installment in the series.
2. The movie must be based on a best-selling book which was released prior to the previous installment.