In act three it seems as if almost everyone is against John Proctor--the judges, the girls, and in the end, even Mary Warren herself. From the moment that he steps in the door, the judges questions everything that he says or does. They are highly suspicious of his motives. Reverend Parris, who is especially against John because John has spoken out against his preaching style, does all he can to turn the judges against him. Parris is insulted by John not liking him, and so makes it his job to discredit John's reputation. For example, before John has uttered more than three sentences, Parris jumps in saying, "Beware this man, Your Excellency, this man is mischief." He then accuses John of trying to overthrow the courts, meaning, to prove that the courts are wrong and thus take away their power to judge in the cases.
The judges then turn on Proctor, because Parris made them suspicious, and because John does indeed appear to be bringing evidence that would contradict their guilty convictions. Danforth even sets up a test, saying if John has only come to save his wife, then, his mission is accomplished, because Elizabeth is pregnant, and hence saved. He tells John to go home because his wife is saved. Later, he sets up the highly emotional and nearly impossible situation of questioning John's wife. Danforth questions everything John does, and then immediately believes Mary when Mary turns against John at the end. The judges are against him because they want to protect their convictions; if evidence shows they falsely convicted people to hang, their careers and livelihood would be over. They have to be all-knowing and powerful, and can't go back on their judgments without looking like fools and evil men.
Parris, Danforth and Hathorne, and even Mary, who accuses John of being "a devil's man" at the end of the act, seem to be against him. Mary turns on him out of sheer cowardice; she doesn't want to be labeled a witch. Abby herself lies in order to protect herself--if she's known as an adulteress who was dancing and casting spells, it is she that will get hanged. John is in a dire situation in Act Three, and it doesn't end well as a result. I hope those thoughts helped; good luck!