How does Francis Nurse show that he is courageous at the beginning of act 3?
Francis courageously storms into the court presuming that the magistrates will listen to him. He approaches them with his own voice of authority claiming that he has proof that the girls were pretending. It is during this time that he also slaps the judges with a biting and sarcastic remark:
Excellency, we have proof for your eyes; God forbid you shut them too it. The girls sir, the girls are frauds.
Not only does this show great courage, but it demonstrates that Francis believes that they might be turning an eye away from the evident truth and that they are choosing to believe these silly girls.
Three times Francis suggests that the magistrates are being deceived. Someone of their stature would take this as a complete insult and he is happy to continue hurling it on them.