I'm gonna try to go about this on the fly. First, it's time to list all the trilogies I know.
Star Wars IV - VI
Star Wars I - III
Back To The Future
Indiana Jones
Pirates of the Caribbean
Rambo
The Godfather
The Bourne ______
Lord of the Rings
The Matrix
Now, obviously, Indiana Jones and Rambo don't count, and not just because there's about to be a fourth movie in each series. To have a trilogy, you've got to have more than the same central character. There has to be a central theme AND a flowing plotline that carries over into each of the movies. Which is why the only trilogies that manage to work AS trilogies are the Star Wars Prequels and the Lord of the Rings threesome (which I would argue are the worst six movies on the aforementioned list, with maybe the exception of The Return of the Jedi and that Indiana Jones flick with Sean Connery).
Here's the thing. The Matrix, The Bourne Identity, The Godfather, Pirates of the Caribbean, Back To The Future, and Star Wars: A New Hope all stand alone as having viable Beginnings, Middles, and Ends. They were made TO stand alone, with the option of creating more, and since each were wildly successful, they got the green light to make however many more they wanted. Which, as it turns out, was two more for each, thus completing the "Trilogy". The thing is, they're not REALLY trilogies, because in the case of each one of these six series (with the exception of The Godfather), you've got one stand-alone film and one extra-long film that was split into two and released one to three years apart.
Now, obviously you could argue that Back To The Future left an opening for subsequent movies to be made, but let's face it, the only reason they stopped at three is because everyone got tired of making more. Each of those other movies though, they COULD'VE stopped right there at one and nobody would've been the wiser.
I'm writing this because sometime in the next week I'm planning on catching up with the last two Bourne movies that I've yet to see. If it's anything like The Matrix, both Star Wars series, as well as Lord of the Rings, I'll be exceptionally disappointed with the outcome, but stranger things have happened. I've also yet to see the last two Pirates of the Caribbean for similar reasons; I heard they were bloated, over-the-top, and not nearly as clever as the first with more money pumped into special effects than quality writing. I hate that shit.
I have higher hopes for Bourne though, since it's pretty much non-stop moving and action from the get-go.
In the meantime, My Kingdom for a fucking trilogy that works AS a trilogy and doesn't suck balls like Lord Of The Shit-Ass Rings.