Common Core Standards
Grade 3
Reading RL.3.9
Compare and contrast the themes, settings, and plots of stories written by the same author about the same or similar characters (e.g., in books from a series).
While this standard says "theme," it may be best to hold off on using this word for now, as themes are not formally introduced until fourth grade. Instead, it may be better to ask students to compare and contrast the messages or morals of different stories, or ask them to think in terms of what each story is generally about (which will sidestep the use of the word "theme").