Comparing "huati", "timu", and "zhuti"
话题, 题目, and 主题 are similar because all are nouns that mean “topic”. 话题 is used in informal situations, whereas 题目 and 主题 are used in formal situations. Each of these three words has distinct uses within specific context. 话题 means a topic of conversation, what you are talking about. 题目 means the title, i.e. the title of a thesis or published work, or in an entirely different context a specific question on a test. 主题 is the theme or subject of a literary or artistic work (poem, painting, etc.).
