Answer: A fable is a short story intended to teach a lesson, a myth is a story used to explain how something came to be, and an allegory is a story that has a hidden meaning, usually about morals.
Explanation:
A fable is something like The Tortise and the Hare, which teaches the lesson that slow and steady wins the race.
A myth is something like how the Greeks believed there was Chaos (nothingness) in the universe before Gaia (Mother Earth) came from light, giving birth to the sky (Uranus), mountains (Ourea) and sea (Pontus).
An allegory may be a story simple on the surface, such as a daughter leaving her mother, but it has a deeper meaning related to morals/the political climate of the time. The daughter leaving her mother may be an allegory for how America is leaving it's morals behind.
I hope this helps!