Queue arguments as to whether it's actually a trilogy.

A few spring to mind:

Back to the Future
The original star wars (but was it a trilogy or a delayed sexology? (which i have a sneaking suspicion is the study of sex and i refuse to google it))
Matrix, but firstly wasn't just a brilliant intro and then two average movies to round up a storyline? and then they made a fourth so it hardly counts, does it?

There are a few more with one very obvious one, but I'm going to save my caveat/opinion for that later.

---