The Third Act is your race to the finish line. Everything has been setup and the final payoff(s) are coming in.

Remember, your Third Act is buried in your First Act. You’ve already raised a question about your Protagonist, placed a significant obstacle (the first of many) in his or her way, and you’ve setup something that has to be resolved. Additionally, this final act resolution by the Protagonist often comes to signify their transformation. (Yes, nothing new here...)

Typically, Third Acts are not more than 15 - 20 minutes long. (There are always exceptions). The event that sends the Second Act hurdling into the Third Act should be the emergence of the Protagonist overcoming his lowest point. The Antagonist is on the threshold of victory. 

The Final Action is taken (engaged) by the Protagonist. Now it's a race to the finish. The suspense, tension and drama are at the highest point during this time.  You have to maxmize the tension and get the audience on the edge of their seat.  What is going to happen?

I don't like to visualize the narrative action line as flat. To me it is rising and rising until that Third Act resolution.

You often will expose the theme of the story with this final act of redemption, resolution, whatever... How and why the Protagonist has acted tells us something about the character. Your Third Act probably should do the following: 

1) Resolve the central action line; 
2) Reveal the final image of the Protagonist---has he/she changed? How? Why? What is this new identity?
3) All that is setup must be paid off now.

Finally, the pace and tempo should be at a steady march here. The Three-Act structure is a building process of emotion and tension. (Note: all kinds of "act" structure types, or ways to look at structure!)

The Protagonist is heading towards this Third Act deliverance where the final decision is made and the action is resolved thereby allowing the theme of the story to be revealed.

Hope this basic explanation helps a few of you, happy writing!