What are the ironic events of Act 3 of The Crucible?

Act 3 of The Crucible is filled with
irony, and usually in a way that is always negative, adding to the conflict and stress
of the play itself.  Irony is typically defined as the opposite of what is expected
occurring.  As readers that are hopefully distanced from the historical events of the
time, and as critical thinkers, there is a whole lot going on in this act that we find
surprising.

First of all, let's look at what John, Francis
and Corey do to try to save their friends from jail and the noose.  The first thing they
do is offer a petition, signed by nearly 100 people (a lot in that size of a town),
attesting to the Christian and righteous nature of their wives who had been arrested.
 Logic would dictate that such a weighty petition would hold credence in the courts, and
that the courts would take pause and reconsider their accusation of witchcraft.  Not so.
 Instead, Danforth has all of the people who signed the petition arrested.  It is sadly
ironic that the people who were testifying of people's righteousness are then asked to
defend their own.  The second attempt is Giles Corey letting the courts know that he has
a witness who heard Putnam essentially admit that he was forcing his daughter to call
people witches, just so he could get their land.  Logic would also dictate that this
charge be taken seriously; instead, Giles is arrested.  Lastly, when John steps forward
to admit his affair with Abigail, we would expect the courts would discredit what Abby
has said and done, because she is a hypocrite and a liar.  Not so.  Instead, in the end,
John is arrested for being "the devil's man."

One last
moment of extreme irony that I would like to point out is when Elizabeth lies to protect
her husband's reputation.  John is so confident about her honesty that he tells the
courts "she hath never lied," ever.  But when they ask her if John is an adulterer, she
utters the first lie of her life, and says that he is not.  This is taken as proof that
John is a liar, and his claim of Abby's wickedness is dismissed.  It is ironic that a
completely honest woman would lie before God and the
courts.

Those are just a few examples of irony in Act 3,
and I hope that helps you to get started.  Good luck!