How is Romeo and Juliet similar to To Kill a Mockingbird and Pride and Prejudice in relation to theme, conflict, and other elements?

In all three works, relations between individuals are impacted by rigid social distinctions between different groups of people.
In Romeo and Juliet, the two young lovers simply can't fall in love and marry as they please because their families are divided by a bitter feud. This creates a number of problems for the lovers and leads to their tragic deaths.
As in Romeo and Juliet, in Pride and Prejudice social distinctions interfere with the ability of the protagonists to pursue love and marriage. Bingley's friends and family discourage his genuine love for Jane, because they know she has no dowry; they also dislike her family, and they don't think she is good enough for him. Likewise, Darcy believes it would be a great comedown to marry Elizabeth, with her lack of money and socially challenged relations. He is willing to overcome this, but is so insulting when he asks Elizabeth to marry him that she refuses. Later, Darcy's aunt actually comes and forbids Elizabeth to marry Darcy, saying she is of the wrong social class. Elizabeth ignores this command, notes she is a gentleman's daughter, and says she can make her own decisions.
In To Kill A Mockingbird, the friendship between Mayella and Tom Robinson is impossible because she is white and he is black. Even talking alone together is forbidden. When Mayella shows a sexual interest in Tom, her father beats her, and the family accuses Tom of rape. The dynamics of the town are such that the whites turn against Tom, even though he is clearly innocent, and against Atticus for trying to defend Tom fairly.
In all three cases, love between two people is thwarted or made very difficult by the social structure in which the individuals live. Thematically, all three works critique an arbitrary social construct: Shakespeare critiques the feud that divides the families and costs so many young lives; Austen critiques a system in which money and snobbery are more important than love; Lee critiques the racism of 1930s Alabama.