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.