The third act is both simple and complex.
Simple in that no new evidence is added – everything used or known in Act Three needs to be, at the very least, hinted at in Acts One or Two.
Complex in that all of the threads splayed out in Acts One and Two need to be neatly tied up, or your reader is going to be pissed off.
If you remember, we ended Act Two, first with an “all is lost” moment, where the forces of evil seem to be unbeatable, followed by the Second Plot Point, that piece of evidence that the hero knows is enough to get him to the finish line, even if it kills him. Or her. I’m equal opportunity.
And now we’re in Act Three.
The interesting thing about Act Three is that the hero can know what the finish line is, and you, as the writer, can throw everything including the kitchen sink at him (her) and they have to take it. Nothing they can do about it. For genres that are heavy with action (the type of thing I write tends to avoid the contemplative hero), you should relentlessly test your hero at every opportunity. And in Act Three, they can win. It shouldn’t be easy, but they can win.
But they/he/she can’t beat the main protagonist first. In fact, for the first part of Act Three, only the hero should know who the real protagonist is. Your readers should think that the toughest of the henchmen is the real protagonist, and when the hero defeats him, then the penny should drop, revealing that the actual bad guy is someone who the reader may have suspected earlier in the book (or maybe appeared to be a straight arrow). A final clue is revealed, one the hero may have known for a little while or one that is revealed in real time. The real baddie is exposed and the hero will fight, to the death if necessary, to right the wrongs.
Along the way, the pieces of the story that haven’t been closed off need to be closed off. And, please, organically. No dropping in a scene to resolve something, when the scene doesn’t make sense. Be a bit more creative than that.
If you’re clever, you’ll have a final battle scene that links back to a scene early in your story – same location, same characters, something that allows you to bookend the story.
Very rarely does the hero die, but when he or she does, it must be a sacrificial death. It must be a death that propels the story to its successful conclusion. A perfect example of the martyr hero is Harry Stamper’s (Bruce Willis) death in Armageddon. Absolutely critical to the successful conclusion of their mission. And a more powerful story for it.
So, enjoy your third act, and wrap the story up in a neat little bow.