¿Cuál es la diferencia entre sus, allí y son?
Su, allí y son son homófonos.  Los homófonos son palabras que suenan parecido pero tienen significados y grafías diferentes.  Su es un pronombre posesivo, como en "su bicicleta es verde".  They're es una contracción de "they are", como en "they're going to the game tomorrow".  Hay es un adjetivo, como en "siempre está ahí para sus amigos".  También puede usarse como sustantivo, como en "no te acerques", o como adverbio, como en "detente allí".
What is the difference between their, there, and they're?
Their, there, and they're are homophones.  Homophones are words that sound similar but have different meanings and spellings.  Their is a possessive pronoun, as in "their bicycle is green".  They're is a contraction of "they are", as in "they're going to the game tomorrow".  There is an adjective, as in "she is always there for her friends".  There can also be used as a noun, as in "stay out of there", or as an adverb, as in "stop over there".