These simple words cause considerable confusion but are not really hard to keep straight. Their is a possessive pronoun: "That is their daughter." There means "in or at that place": "I was there when she arrived." They're is a contraction of "they are": "We are disappointed because they're not going with us." There's is a shortened form of "there is": "There's money in the bank."
Dictionary of problem words and expressions. Harry Shaw. 1975.