Let's talk a little more about your basic three-act dramatic structure using an example most of us are probably very familiar with: the original Star Wars trilogy.
At its simplest, the three-act structure consists of:
I. Introduction
II. Complications
III. Resolution
The Star Wars movies are fun examples because everyone knows them and their structures are very obvious to anyone that cares to look for that sort of thing.
Let's look at Star Wars, the first movie, also known as A New Hope:
Act One takes place on Tatooine. We are introduced to all the key characters. Not only that, but right away- in the very first scene-- we see space ships, laser guns and robots. This establishes the kind of world the story will take place in, thereby setting the storytelling rules. This is vital. Because this is established early, then we don't blink an eye when a little while later we see strange alien creatures and a laser sword.
Imagine our reaction to a lightsaber thirty minutes into a movie like The Godfather or Beverly Hills Cop. It wouldn't work because of the storytelling rules set up at the beginning of the film. This is my main problem with Field of Dreams, which everyone else in the world seems to love. The supernatural element of the appearance of ghosts happens way, waay too far into the movie and there's been no establishment of those sort of "rules". To me, it's like Don Corleone using the Force. It doesn't work for me and, so, I hate the movie. I can't get into it.
Regardless, in A New Hope we not only firmly establish the milieu and genre, but we are introduced to all the key players while we are on Tatooine, the desert planet. The first Act ends in the climax of Luke, Ben and the droids escaping the planet with Han and Chewie on the Falcon. See how obvious that is? They leave the planet, end of Act One.
Act Two takes place on the Death Star as they seek to free Princess Leia. Again, it builds to a climatic scene in which Ben is killed by Darth Vader. This also functions as a "pinch fight", meaning it is a direct conflict with the main antagonist serving as a way to build tension toward the final Act's showdown. They leave the Death Star, end of Act Two.
Act Three is the trench battle around the Death Star. Of course it ends with the Death Star destroyed and the Good Guys win the day. But an important element in making this ending work is that it does involve a showdown with Darth Vader. Vader is established as the Main Bad Guy in the opening scenes of the movie, there is the "pinch fight" with Ben at the end of Act Two and therefore there must be a final showdown at the end of the movie. There must be, that's how it works.
Quickly: the other two movies. The Empire Strikes Back. Act One: Hoth. Act Two: Dagobah and Bespin. Act Three: Luke on Bespin.
The Star Wars movies are truly that simple in discerning the dramatic structure. In Return of the Jedi, Act One takes place on Tatooine, at Jabba's Palace. Act Two takes place on Dagobah and Endor. And Act Three is a rousing three-way climax between a spaceship dogfight battle, a ground battle on Endor and Luke's lightsaber fight with Vader.
But what I like even more about the trilogy is that you can "zoom out" so to speak and see all three movies as telling a single Three Act story, each movie serving as one Act in the play.
A New Hope: Introduction
The Empire Strikes Back: Complications
The Return of the Jedi: Resolution.
I hope to follow this pattern, to some extent, with my novels. Each book needs to stand on its own, structurally, but also I want the three novels together to serve a larger Three Act story.
A couple of more items: the final act's resolution needs to be true to your theme. The theme in Star Wars is one of redemption. When Darth Vader finally overcomes his evil ways and throws the Emperor to his death, he is redeemed. That final showdown is resolved in a way that deals directly with the idea of redemption.
Also, I believe a story is better served when there is not only an external conflict (the Empire versus the Rebellion) but an internal one as well (a belief that there is still good in Anakin). It is the internal conflict that often makes the main hero more relatable to the audience. The final act's resolution should address both of these. Again, this is something I hope to accomplish with my books.