what three reason does Atticus give as to why he must defend Tom Robinson?

As Atticus has explained to Scout many times about this, He is doing this becasue he values equailty and justice. He believes that everyone is equal and therefore just because Tom Robinson is black, doesn't mean that he should not defend him. He says to Scout many times, if he did not defend this man, he could never tell Jem or her (Scout) not to do something. He has to set a good example for his children and if he expects them to do something or follow certain values, then he must display them first. In this case, he is displaying that everyone deserves a chance and that everyone is equal. He is also trying to make the town of Maycomb more accepted of this idea since this is a town of great discrimination. Even though he is only one man, he is hoping to cause a ripple effect and hoping that people will see the light and start believing that everyone is truly equal, no matter what their skin color is.