In Act 3 of "The Crucible," what are three instances of irony?

1.   The near 100 people that signed a petition in the hopes of testifying to the good reputation of some women accused as witches were themselves all "arrested for questioning."  All these people wanted to do was let the courts know that they knew

"the women many years and never saw no sign they had dealings with the devil."

Ironically, these people, in an attempt to save these women from the accusation of witchcraft, are all themselves arrested.

2.  The one man that was trying to help Mary Warren do the right thing, and to turn back to piety and goodness, is the one man that Mary ends up turning on and accusing of being "the Devil's man."  John Proctor, who knows that the girls are faking, has spent a week at home with Mary, encouraging her to do the right thing, helping her to reject the lies, and giving her the strength to go to the courts and confess, ends up being her victim.  At the end of the act, Mary turns on John, and accuses him of being a witch himself.  One wouldn't expect the one person that was trying to help her to be good to be turned on and called a servant of the devil.

3.  Elizabeth Proctor, who his husband asserts "cannot tell a lie," tells a lie.  She lies about John's affair, claiming that he was not a lecher (adulterer).  One would not expect Elizabeth, who has a reputation for honesty, and who chides her husband for not being forthright, to be honest in the courts. This instance of irony is certainly understandable--she was trying to protect her and John's reputation; it's just unfortunate timing.

I hope that helped; good luck!