You can pretty much guarantee in Hollywood these days that, if a blockbuster film is successful, not just one but two follow-ups will be on the way. Yes, in these times when the word "trilogy" is bandied about pretty freely, it's not the sequel but the threequel* that you've got to look forward to as well. The problem here is that often there isn't enough material to sustain three films or the second and third are rushed into production together (increasingly common) and so don't have the same amount of care taken over them as a result. This can lead to a trilogy limping to an unsatisfactory conclusion. I know what you're thinking; that so far, this is all idle conjecture on my part without any proof to back it up. Well then, let's take a look at some of the main offenders, shall we?
Pirates Of The Caribbean - On paper, the first film was something that really didn't have any right to work at all. An old-fashioned swashbuckling, pirate-y adventure yarn which is based on a theme park rife of all things? Did nobody remember the lesson of Cutthroat Island with Geena Davis? Fortunately not, for the first film turned to be brilliant - a good fun family romp very much in the mold of Indiana Jones with a top scene-stealing turn from Johnny Depp. So inevitably, there would be sequels. And sadly, they went for the back-to-back filming route. The next two films up the complexity and the running time considerably (the third film clocking in at juts under three hours) but at the expense of a lot of of the charm that the first film had. There are still some great moments in these last two film but, by the point of the third film that everyone is quadruple-crossing everyone else for barely comprehensible reasons, I kind of stopped caring a bit.
The Matrix - The first film was a breath of fresh air when it came out. Sure, it notions of "what is real, anyway" are nothing particularly new to sci-fi (Philip k Dick had spent most of his career asking that very question) but it combined it with a very distinct visual style and some great martial arts to make something memorable. So along come the inevitable filmed-back-to-back sequels and the returns on these diminished so much that, by the damp squib of a denouement to the final film, I was left feeling annoyed that I spent so much time watching something that turned out to be so anti-climactic. (Although I quite enjoyed the second film - I missed it at the cinema so, by the time I caught it on DVD and everyone had told me how awful it was, my expectations were lowered to a point where I could enjoy it. The third one's still crap though.)
X-Men - Oh, it all started so promisingly. Build-up and scene setting in the first film, an escalation in the second film (one of the few times that a sequel is better than the first film) and then the director jumps ship to direct Superman instead and someone takes all that lovely build up, pisses on it then throws it in the bin. Seriously, rarely has a franchise been squandered so badly. Characters are discarded with little care or attention, plot lines that have been brewing in previous films are pretty much dispensed with and Vinne Jones plays Juggernaut. It's quite insulting, really. And it also contains one of the worst continuity mistakes I've ever seen where it switches from day to night in the middle of a scene. I'm not just being a picky film buff here - the person I was with in the cinema actually turned to me at that point and asked if that just happened.
I could go on and I'm sure you can think of many examples yourselves but you get the gist with that anyway. Hollywood - sometimes, all it needs is one film. Give it a rest, eh?
* Horrible word, isn't it? Nasty little marketing term that's meant to sound cute and clever but actually just sounds smug and irritating.