There are two basic problems with third acts.
The first problem I find is that when my students are structuring new screenplays, there is always a concern about whether or not they need to know the ending before they begin to write. My reply is always that they already know the answer: the ending will be happy, sad - or something else. I suggest they write the scenes they are sure of and the end will become apparent.
Also if the writer has created juicy, dynamic characters, they grow and change during the writing process. If the writer forces the characters to behave in a certain way in order to fit the ending he or she has planned, this can dilute an otherwise strong piece of work.
Writers who are willing to be open to what happens in the third act write better scripts. There will be greater suspense because the writer doesn't know what the outcome will be.
The second problem I find is that writers often don't answer the dramatic questions in Act 3 that they have set up in Act 1.
Bill Wilder said, "If you have a problem with the third act, the real problem is in the first act."
And he was right. If your third act isn't working, don't blame it. The trouble is probably back in the first act. The first step is for you to go back and look at Act 1. What expectations have you set up for your audience?
Now review what happens in Act 3. Have you really answered those questions and satisfied your audience? If not, you know what to do.
For example, in Witness, the first act raises questions: will John book catch the killer, will Samuel and Rachel escape and will Rachel, a widow, find love?
The third act, answers these questions: the bad guys are caught, the boy is safe, and Rachel will find love, but not with John Book.
By identifying the dramatic questions you have set-up in Act 1, and pay them off in Act 3, you will definitely create a strong third act.