I'd add to those three themes the age-old one of "loving your neighbor." "Prejudice and tolerance" might be another way to phrase this theme.
The novel is published in the Civil Rights Era and addresses racial prejudice, so it seems very much a statement (even if not, to my taste, not an entirely progressive one) that all people should be treated fairly, regardless of skin color.
The plot lines involving Tom Robinson (an innocent man who's tried and convicted because he's black and his accuser is a white woman claiming rape) and Boo Radley (a neighborhood shut-in) seem to me to be structured entirely around this theme of "loving your neighbor."
Three apparent themes in TKAM are: maturity and growing up; good vs. evil; and courage in the face of adversity. Maturity can be seen clearly through the characters of Scout and Jem, as we as readers follow them through their early childhood and watch them mature through the events of the novel. Good and evil can be seen in the struggle between Atticus and the blind racism of Maycomb; Atticus and Bob Ewell (trial); Boo Radley and Bob Ewell (climax of the story). Courage in the face of adversity can be seen in Atticus’ decision to defend Tom Robinson in the trial despite the fact that his decision will be an unpopular one in Maycomb; Mrs. DuBose kicking her morphine addiction; and Boo Radley saving Jem and Scout from Bob Ewell.