How does Arthur Miller create tension in Act 3 of The Crucible?How would I write a good introduction with this question?

Miller masterfully builds suspense and tension in Act 3 through the following elements.

1.  The judges and "afflicted" girls for once in the play have a worthy opponent in John Proctor.  He comes to court armed with Mary Warren (who can but doesn't testify that the girls are faking), a petition of sorts testifying of the goodness of his wife and Martha Corey and Rebecca Nurse and signed by many of the townspeople, and a willingness to confess his adulterous relationship with Abigail.  It seems that the tide might change in the trials if John is successful.

2. The tension between Abigail and John is at is most elevated level in Act 3.  Abigail knows that John might tell the court that she confessed that the dancing and conjuring in the forest was just for "sportin."  She is aware that he has the power to end all the attention that she has been receiving.  Likewise, John knows that he must make a choice at this point between keeping his pride and reputation and his wife and others' well-being.  Only Abigail stands in the way of his wife being released from jail.

3. In Act 3, the tension also mounts between Rev. Hale and the judges.  Rev. Hale, who was originally on the judges' side, has come to see the truth of the situation in Salem and enters into a conflict with the judges and Rev. Parris.

As the playwright draws Act 3 to a close, the audience is left wondering what actions/words will resolve all of the conflict and tension.

In answer to your question about an introduction, I am not sure if you are going to turn the question into a thesis statement, but if you do so, then you should begin your introduction with a broader discussion of conflict/tension in general or perhaps even several sentences about how the quest for power leads to tension in a community.  Good luck!