For some reason, it’s very hard to nail the finale of a movie trilogy.
For example, X-Men: The Last Stand is a pretty underwhelming third film, and if you want to be really technical about it, it did conclude that initial X-Men movie trilogy.
Picking the worst final thirds of movie trilogies is tricky, though, and not just because there are so many candidates to choose from.
Part of the reason why could just be a matter of probability.