Third times the charm?

I saw X-Men III, and give it a solid "it was ok" and if I'm honest with myself I just didn't like it. I can't get into why without spoiling the surprises (happy to do so in the "comments" section to spare those who haven't seen it), but it missed. However, I have to say I was a major, major comic book geek and the Uncanny X-Men was like my crack, but I really liked the first two. This one was by far the weakest one of the trilogy.

Which got me thinking about treequels. Why is the third one always so bad? There's no excuse for X-Men, where there's decades of story lines to explore. But I also saw Mission Impossible III, the worst of that series. Start Trek III, terrible. Jaws 3 in 3-D, oh man was that bad (despite Dennis Quaid's efforts), Return of the Jedi was third in that group, Once Upon a Time in Mexico (third in the El Mariachi, Deperado series) was borderline unwatchable, Superman III (Richard Pryor couldn't save that mess), Forget Highlander III (talk about killing a great original movie), the list goes on, and on, and on.

Makes me really worried about Spiderman III (a fantastic series), the heroin that fed my comic geekiness.