What are 3 of the most significant examples of courage demonstrated in To Kill a Mockingbird?

Three significant examples of courage demonstrated in To Kill a Mockingbird are (1) the stand-off at the jailhouse with Atticus and the Old Sarum Bunch, (2) the appeal by Atticus to the consciences of the jurors as he exhorts them to follow the Constitution and their consciences, and (3) Arthur Radley's heroic defense of Jem when he is attacked by Bob Ewell.
1. Jailhouse stand-off
When a number of cars with the Old Sarum Bunch pull up, Atticus, who sits in an office chair in front of the jail where Tom Robinson is held, calmly folds his newspaper and stands up. Mr. Walter Cunningham tells Atticus, "You know what we want." Atticus suggests that they leave, saying, "Heck Tate's around somewhere"; however, one of the men contradicts Atticus: "Heck's bunch's so deep in the woods they won't get out till mornin," implying that the sheriff has falsely been called away.Although his hands shake when he hears that the sheriff is gone, Atticus's voice remains calm. "Well then,...that changes things." But, Atticus does not move out of the mob's way.
Fortunately, Jem and Scout, who have come to find their father, refuse to go home and after Scout speaks to Mr. Cunningham with such words that remind Mr. Cunningham of the kind acts of Atticus toward him, he tells his men, "Let's clear out....Let's get going, boys."
2. Atticus's appeal to the jurors to follow their consciences and the Constitution
After all the vilifications about Atticus by citizens in the town, as well as his acquaintance with the conventions of the townspeople, it is a courageous man who makes an appeal in court to the jury before the citizens of Maycomb that contradicts their beliefs and culture. Reminding the jury of the American courts' being the "great levelers" in which "all men are created equal," Atticus addresses them:

"Gentlemen, a court is no better than each man of you sitting before me on this jury....I am confident that you...will review without passion the evidence you have heard, come to a decision, and restore this defendant to his family. In the name of God, do your duty."

3. Arthur Radley's act of bravery
When Boo Radley hears what goes on outside his window, he apparently investigates what has caused the Finch children to sound distressed. Without hesitation, then, he rushes outside, wrestles with their attacker, Bob Ewell, and the vicious man is stabbed by his own knife. After saving the children's lives, Boo carries the children home. It is a grateful Atticus Finch who says, "Thank you for my children, Arthur."