I think that the primary motivation behind the third act was to explore the level of personal and political injustice that existed in Salem at the time of the Witchcraft Trials. Act III is the first time we actually see the trials, themselves. Miller has to bring out the courtroom in order for the reader to fully grasp a couple of things. The first would be the fradulent nature in which "spectral evidence," evidence that consisted of visions, seeing demons and ghosts, and supernatural phenomena were allowed to be used against those accused. Miller is able to display this with Abigail seeing "the yellow bird" in the courtroom to contradict Mary Warren's testimony. At the same time, Act III allows the reader to really understand how the "trials" were actually fraudulent in that they did not allow for full representation for those accused. It also gives insight into how flimsy the cases against many of the accused actually were. In the forms of Judges Hathorne and Danforth, Act III is able to fully show how those in the position of power in Salem were more concerned with substantiating their own position than any real adjudication of justice. The reader does not see this in any other point except in Act III. Finally, it is the moment when Proctor can no longer be silent. Throughout the play, Proctor had been shown as someone who understood what he needed to do, but lacked the initiative to do it. Act III is the moment when he has to confront Abigail once and for all and also face down the hypocrisy of Salem, something that he had been willing to live with for some time. Act III is the moment where we see Proctor transformed from an ordinary man into someone of extraordinary capacity, a transformation that will be completed in Act IV.