Each act has particular function; if you miss the third act then you do not complete all functions and therefore you do not complete the story. Beginning, middle and end is not an arbitrary expression.
There are solid reasons behind a third act, which include:
The first act involves pulling away from the Ordinary Self, the second act involves becoming the New Self and the third act involves pulling away from the New Self and merging the Ordinary with the New to become a Mature Self.
When you enter Act II, you are not ready, when you exit Act II you are ready. One function of Act III is to return and demonstrate capabilities gained / change by suppressing that which could not previously be suppressed.
Within Act II, limitations to your return exist. Exiting Act II, limitations to the return are overcome.
You transcend out of Act II physically (a hard break) or psychologically (a soft break).
A physical break will, commonly, involve exit from one world and entry into another world (these do not have to be completely new worlds, they may have been visited before - which may make the break appear seamless).
A psychological break will, commonly, involve exit from one-way-of-thinking / limitation or similar. This can also add to the illusion of seamlessness, if the break is not clear.
There are many more reasons why a third act must exist. You would be wise to learn about them
This is not impractical, abstract theory, it tells you HOW TO WRITE YOUR STORY/SCREENPLAY. Consider The Godfather (1972). In Act I Michael is drawn away from his Ordinary Self. By the time he returns from Sicily he is a New Self, a young shadow of his father (limitations to his return have been removed due to the oath his father has taken). In Act III, Michael replaces his father. It is your duty, as a writer, to comprehensively understand what a third act is all about and what functions need to be performed within it.
More info / buy the 2000+ stage Hero's Journey And Transformation Through A New World / State at http://www.clickok.co.uk/index4.html or http://www.kalbashir.com