Final answer:
A trilogy is the term for a group of three plays or books with a common theme, combining the Greek 'tri-' for three and '-logy' for story.
Step-by-step explanation:
The word related to the Greek or Latin root/affix that is defined as a group of three plays or books with a common theme is trilogy. The term stems from the combination of the Greek prefix tri-, meaning 'three', and the suffix -logy, coming from the Greek word logos, meaning 'story' or 'speech'.
A trilogy is a common structural form in literature and may be found in works ranging from ancient Greek plays to modern novel series. For instance, the Oedipus Trilogy by Sophocles is a classic example of a trilogy in Greek tragedy, which imparts the story through dramatic action and heightened language intended to elicit pity and fear, ultimately leading to catharsis for the audience.