The meanings of these three words are very similar, and all relate to creations of the imagination. But there are some nuances. If it’s in a negative context—not being factual for the purpose of being deceptive—use “fictitious”. If you are discussing literature, use “fictional”. “Fictive” is less common than the other two, and is often used in the phrase “fictive kinship” which is a relationship based not on familial ties but on a close relationship. (e.g. when you call your mother’s best friend Aunt Sue)
