Not a Trilogy
David R.

LOTR is actually divided in six parts, so there should really by six movies instead of three. The usual line-up with three books is probably just for convinience. My edition contains all parts, and still doesn't make my hands stiff.

I would say that I agree with Laura about LOTR not being a trilogy, even if you don't consider how many parts it consists of. I would define a trilogy as three separate works with something in common.