Evil Dead rule
Rule inspired by Sam Raimi's Evil Dead trilogy in which the first film is great, the second is really great, and the third is a total stinker. One of the other trilogies to follow this rule was Raimi's Spider-Man series. The first three X-Men films - before the franchise began expanding into many different directions and timelines - also fit this rule.
The rule is opposed to the High School Musical rule, in which it is the second film that lets down the series.