What three characteristics does Atticus exhibit during the courtroom scene in Harper Lee's To Kill a Mockingbird?

The courtroom sequence is perhaps one of the most significant scenes in Harper Lee's To Kill a Mockingbird. Among other things, the scene gives an intimate insight into aspects of Atticus Finch's personality. To answer your question, I'll focus on the three aspects of Atticus' character that I believe come most prominently to the foreground: intelligence, courage, and kindness.
Intelligence: it's difficult to finish reading the courtroom scene without marveling at Atticus' intelligence. During this scene, Atticus not only shows off an eloquent insight into race relations and the role of the legal system in American society, but he also exhibits the ability to discern the truth through quick-witted questioning.
Courage: It goes without saying that Atticus' decision to defend a black man in a court of law is unpopular in Maycomb. Indeed, throughout the book, Lee shows Atticus being ridiculed by his neighbors for refusing to adhere to Maycomb's underlying racist culture. As such, Atticus' determination to earnestly defend Tom Robinson in court is a remarkable act of courage.
Kindness: While Atticus is determined to defend Tom Robinson, he avoids descending into a mean-spirited attack when questioning the Ewells. Indeed, Atticus treats Mayella with respect and kindness, and it's clear that, though he wants to expose her lies, he also does not want to needlessly humiliate her. As such, Atticus proves that an authentic kindness lies within his occasionally stern exterior.