Can you write down 3 themes that are clearly presented in the novel To Kill a Mockingbird?

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."