Explain how, in Act 3 of "The Crucible," how characters are able, or not able, to defend themselves against the charges. "In an ordinary crime,...

According to that reasoning, you cannot defend yourself against that sort of accusation.  Consider all the ways that, in Act 3, the characters tried to disprove the accusations: The petition brought to the courts stating the Christian character of the women accused, Giles' testimony against Putnam, John's confession of adultery to try to disprove Abigail's testimonies, and even Mary attempting to discredit her friends.  None of that "other evidence" worked, and all else was overridden when Abigail started crying spirits in the room.  And as soon as Mary turned on John, saying he had come to her as a witch to turn her to Satan (very end of Act 3), the courts believed her above all else.  Even Elizabeth knowing her commandments (Act 2), and Rebecca Nurse being revered as almost saintly (look at Hale's greeting to her in Act 1) did not provide evidence against accusation.