Miller uses a great deal of dramatic irony to build tension and suspense at the end of act 3. Abigail Williams and Mercy Lewis pretend to freeze as a result of a "wind, a cold wind" in the courtroom, for which they blame Mary Warren. They accuse her of witchcraft, and the magistrates believe these lying girls rather than Mary Warren and John Proctor, who are telling the truth. We know that the girls are lying when they accuse Mary and that she is actually the one being honest, but the magistrates seem to believe the girls rather than the honest people, and this creates dramatic irony.
Another example of dramatic irony building tension is when Elizabeth Proctor lies to the court to protect her husband's reputation, but Deputy Governor Danforth believes her lie rather than John's truth. Again, we know that Danforth believes the wrong thing, and this builds suspense and tension for us.
One of the best techniques used is Proctor's sarcasm in the lines:
Proctor: laughs insanely, A fire, a fire is burning! I hear the boot of Lucifer, I see his filthy face! And it is my face, and yours, Danforth! ... We will burn together!
The magistrates have just asked Proctor for his confession a "final" time and he is giving it. He doesn't mean it though. He is acting just like the girls and he is "seeing" something fake.
We also see characters turn near the end of Act III. Thus, they are being dynamic characters. Mary Warren who had come to court on behalf of Proctor, is now back with Abby, and Hale who had been on the side of the magistrates in the beginning is even damning the court.
Finally, I think John Proctor's line:
You are pulling Heaven down and raising up a whore!
creates great conflict. This is a true statement, it is metaphorical, and it attacks the very core of their beliefs. He is trying to show their hypocrisy.