Why is Act 3 of The Crucible described as the most dramatic?
I think that a case can be made for any of the acts containing drama. The first act with the accusations being flung around at its end is extremely dramatic as it sets everything in the drama in motion. The second act's drama lies in the tension between Elizabeth and John, and her arrest, with Hale being both helper and agent of resistance. The final act is powerful in Proctor's evolution before the audience's eyes. This play contains drama everywhere.
I think that the primary reason why Act III can be seen as the most dramatic is because it represents the "showdown." In all such moments where there is a showdown of sorts between protagonist and antagonist, drama is present. Act II closes with Proctor screaming about how everyone is "naked" in the "icy winds" of judgment and how if Abigail is going to pursue her accusations, he will proceed to expose her as a fraud, even if that means he will suffer as a result. It also ends with Mary Warren weeping that she "cannot" face Abigail and accuse her. It is within this framework where the reader recognizes that one way or another, this drama will unfold. In Act III, the tension rises also because of the configuration of how the courtroom operates. Danforth and Herrick are convinced in the authenticity of the court procedures and in what they do. Along with the accusations and the defense lobbied by their loved ones, this adds to the tension.
The idea that those who are brought in front of the courtroom must follow court procedure in order to clear their names helps to add drama. Those accused must disavow that they were ever witches, confess their love for accepted religion, and then, in order to prove their faith, accuse others for crimes they may or may not have committed. It is not merely good enough to declare innocence in such a setting. One must declare, repent, convert, and then accuse in one setting. This, in its own configuration, is dramatic because it involves so much in terms of conflict. Giles Corey is thrown in jail because of his refusal to follow such a code. Francis Nurse sees the 91 people whose names he sought in defense of his wife and Proctor's wife all become suspects. Proctor himself cannot bring himself to admit that Parris is a religious man, nor can he stand to see Abigail and her crew tear down Mary. His intense reactions to her testimony in this legal setting only add to this drama.
This, of course, bring the height of the drama to the forefront in Abigail Williams. Interestingly enough, she does not appear much in the play, but she is the catalyst for everything in it. Abigail's presence is dramatic. When she fakes seeing the devil in the courtroom, mocks Mary, gets the other girls to mock her, and then play innocent, there is drama present. When she threatens to leave the courtroom and then portray herself as a victim, something that Danforth believes, to an extent, and John derides, calling her a "whore" repeatedly, drama is present. The moment of intense drama has to come when the triangle of John, Abigail, and Elizabeth is constructed, where the wife can only see the backs of her husband and his lover. In an instant, she lies to protect the name of her husband, a name that he himself has already sullied in the attempt to bring down Abigail. In this, there is drama. With the en of the act, Proctor has declared God to be dead, and there is intensity all around. That's fairly dramatic.