What does Hale mean when he asks if every defense is an attack upon the court, and how does this show that Hale has changed by the end of act 3?
Hale, in Act III, goes through a profound change.
First of all, in Act III, the court is going through the proceedings for many of the women in the village. This is when Proctor stands up to Abigail, tries to get Mary to testify against Abigail and the other girls and finally confesses to the court of his affair with Abigail. Mary accuses the women of being false, but Danforth is not convinced. At Proctor's accusation that Abigail is a whore and a liar, Danforth calls Elizabeth Proctor to the courtroom (from the jail) to get her to verify the affair. Elizabeth does not want to sully her husband's name, so she denies knowledge of an affair.
Hale, however, sees through Elizabeth's lie and tells Danforth that he always thought Abigail was not truthful. Suddenly the girls start screaming and pointing at Mary, Mary finally, with no choice other than to rejoin the hysterical group, starts a fit as well. Suddenly it is not Mary's spirit upon them but John Proctor's. John is accused of witchcraft.
Finally, Hale denounces all the proceedings in the courtroom. He cannot be a party to the madness any longer. He sees the witch trials as what they really are: a ploy for attention and power.
The quote is "every defense an act against the court" refers to the fact that any person who tries to defend him or herself or a loved one is immediately accused of being against the court. Danforth takes it as a personal attack and an attempt to vie for power. Hale wonders aloud if this isn't a bit paranoid of Danforth and the court to assume that everyone defending themselves is attacking the court.