[Xantar]

It's not just videogames. The movies are doing it, too. The next X-men movie will probably be the last one. Batman is set to be a trilogy. And of course, we had The Matrix which was apparently conceived as a trilogy from the start.

Of course, that happens in the movies because actors sign three movie contracts as standard practice. It's happening to videogames either because they are imitating the movies or because that's how many developers figure they can put out before people get sick of the franchise.

Although on the other hand, lots of videogames go way beyond trilogies. Just look at all the big names in the PS3 lineup.