Threequel is a word that can be used to describe any third part in a film franchise. However, a richer understanding of threequels would highlight that they are third parts that exist primarily because of financial rather than narrative considerations: they are made because there was more money to be milked, rather than a need for a closing chapter in a planned three-movie arc. Therefore The Lord Of The Rings: The Return Of The King isn't necessarily a threequel in the way that Rush Hour 3 definitively is.
The watershed year for threequels was undoubtedly 2007, in which the following films were released (in chronological order):
- Spider-Man 3
- Shrek The Third
- Rush Hour 3
- Resident Evil: Extinction