What various techniques does Arthur Miller use to achieve such heightened drama at the end of Act III?
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.