Act 1 and Act 2 exist only to set up Act 3.
There is a beginning, middle, and end to your third act.
Here, milk your suspense. Raise the audience's level of anticipation.
Less dialogue will always work better in your third acts.
The Beginning of The End
Act 3 begins with the preparation to implement the decision made in Act 2.
Usually, the third act preparations are active/actions.
(EG. In The Glass Hammer, Cliff preparing for the escape, Elliot rushing to rescue E.T.)
Sometimes, these preparations will be a mixture of action and reaction to events happening to the protagonists. (Like Butch and Sundance)
The Middle of The End
The middle of Act 3 will be the action itself.
(EG. The breakout of the hospital in The Glass Hammer. The capture of the pedophile in Fallen Angel.)
This action, be it emotional, physical, or a combination of the two, is exactly what the entire movie has been building to and leading the audience toward: The Accomplishment. The Goal.
The End of The End
The most memorable movies promises the future of the protagonist during its last few frames.
(EG. Butch and Sundance freeze-framing suggesting immortality for the men. Eliot and his friends levitating into space with E.T. In The Glass Hammer, after their literal escape, the audience will project the happy future of Cliff and Susan on an idyllic Idaho mountaintop scenery.)
A balance between these two concepts (Writers who think only in terms of 3 page scenes) and (less is more) will produce the best first draft quality.
Try to come in short rather than long.
Short just means you have to add, which is not a problem. You can always find more passion, drama, and tension in scenes throughout the script. The objectivity of time will also show you scenes you can add into the script, to heighten drama or comedy.