In Act Three, Reverend Parris tries to install himself as a deputy to the court. He has no official position within the court, but he is clearly trying to get on Deputy Governor Danforth's good side while also trying to strengthen his own position and power. To Danforth, Parris says of John Proctor, "Beware this man, Your Excellency, this man is mischief." When Proctor explains his purpose in bringing Mary Warren, Parris exclaims, "They've come to overthrow the court, sir!" He tries to influence Danforth, and he also seems to hope that he can gain more power in order to punish his enemies. He accuses Proctor, in front of Danforth, of not coming to church because he knows this is damning. He also tries to call Proctor out, implying that he doesn't read the Bible either. Finally, he repeats his accusation that Proctor has "come to overthrow the court!" He tries to "enlist Danforth's sarcasm" by employing sarcasm himself to insult his parishioners, and he encourages Danforth to arrest everyone who has signed the petition that attests to Rebecca Nurse's, Elizabeth Proctor's, and Martha Corey's good character.
It is clear that Parris is actively trying to turn the court against John Proctor because he fears the damage Proctor could do to the trials: if it comes out that the girls are lying, then Parris is implicated either as a fellow liar or an incredible idiot. He claims, "All innocent and Christian people are happy for the courts in Salem! These people are gloomy for it." He implies that anyone who questions or presents concerns about the trials is unChristian or guilty.
Act Three of "The Crucible" is where the action of the court trials actually begins for the reader/audience. We already know that Parris is going to be on the court's side because that will keep any suspicion off of him and keep it on the people who have been accused by the girls. In this act, Parris tries as hard as he can to discredit anyone who tries to go against the court. At times in the act, he even tries to push his way into pretending that he is one of the judges by questioning the accused -- he does this until Danforth finally gets annoyed by it and tells him to stop. This is the role that Parris plays in Act Three.