Epics tend to be in three parts. The notion of the trilogy is often associated with ambitious works of fiction. These are the sorts of works whose far-reaching ideas and rich casts of characters couldn't possibly be encompassed in a single volume.
Trilogies are well known in fantasy and science fiction.
It's true that the official expansion to last year's highly acclaimed role-playing game isn't a true sequel, and, as such, doesn't really constitute a third installment in a trilogy. If the incentive of a grand finale to the story of Baldur's Gate weren't enough, the forthcoming expansion also has a lot more to offer in its gameplay. Most notably, it'll let your characters reach a godlike level of experience. Throne of Bhaal and to see whether this expansion would be everything that fans of the Baldur's Gate series could ask for. We weren't particularly surprised to find that, indeed, it seems like it will be.