Whenever I think of the answer to this question, I always lament at how many film trilogies or games could have been absolutely immaculate duologies but were, for various reasons, sort of forced into a third installment through fan expectations, studio pressure, or just plain Hollywood/corporate greed.
It usually begins with a film or a video game that is an unexpected success, something that was written off by the execs that turned out to be not just a work of art, but a pop culture sensation. Star Wars, The Matrix, The Terminator, Assassin's Creed, Mass Effect, etc were probably never intended to have a sequel. Their original plotlines all tied up the loose ends nicely and made for a perfectly adequate self-contained story. Then the second film/game in the series comes out and it's another well received installment. Maybe it's because the second rides a bit on the coattails of the first, or maybe because the first walked so the second could run, it's hard to say, but in every case the second always sets the bar too high. The third installment is typically the one that sours the soup, so to speak. I'd wager that even a really well written story can't really live up to the expectations that fans have for the third installment of a well-beloved series. Having the perfect three-peat is a feat rarely seen accomplished, but nobody ever seems satisfied with just two good pieces of media with no plans for a third.