The trilogy arc is one of the most common, and theoretically satisfying, narrative devices in use. In our modern day cinema world, we often measure storylines in trilogies, with movies aping the success that Star Wars and Indiana Jones were able to achieve in creating money-printing franchises.
The problem is that writing a trilogy is hard. Too often, the best part comes right in the middle, as that is where you are allowed to be a bit more subversive with your story. The hero can fall, characters (especially mentors) can die, and you can let plot lines hang until the finale.