What needs to happen in ACT III?
1. Your main character makes a decision after the momentary lapse of will at the end of ACT II and starts to move toward his goal with even greater resolve.
What is his goal? That which his opponent doesn’t want him to get? Too simple? Make it as complicated as you want it to be, but those elements have to be there otherwise the third act won’t work.
2. The final battle / climax. The main character faces his opponent. Either he wins or he loses — physically, mentally, or spiritually (or all three).
3. Resolution. The French call it Denouement. How will your character behave now, having gone through all the trials and tribulations you’ve created for him? He can’t be the same person, otherwise what’s the point of telling the story? Some filmmakers will argue with me. But I will argue with them.
4. You shouldn’t have more than two scenes after the final battle. There are movies with several scenes after the climax and they work, but there are not very many of them. Most don’t. The power of the final battle is lost if you continue the film. It just becomes a bump in the road as opposed to a head-on collision.
There’s a book on the market called The Third Act. It’s a worthwhile read. Attached is a chapter from that book to whet your appetite.