There, their and they're are not interchangeable. More often than not when I read a message on Facebook, there's a word misused and then another person attempts to correct it. Usually the word is corrected wrong, too. In high school, my English teacher shared a wall poster created by one of the upper classmate for a school event. The poster read, "Their is a program after school for the handicapped.
There needs will be discussed and donations taken for they're athletic equipment. If you can help, bring contributions to the auditorium." In a red felt tip marker, a student had crossed out the first "their" and scrawled in "they're." The class got a good laugh, and the instructor laughed with us, then asked us to write a response to the posted, using all three words correctly. In third grade, my grandparents had played "their" game with me and taught me the difference between, they're, their and there.
I spent the better part of the next thirty minutes sharing the difference between the three words, and correcting the poster. There now... There, refers to a place, explained and identified by the 'e' on the end of the word, matching the 'e' on the end of the word place. You can't forget that over there is a place and there is an 'e' on the end of there and place. Their, the possessive form of the word contains the letter 'i' thereby identifying the word as personal, as in belongs to them.
Theirs. Anytime the word their has an 'i' in it, it becomes a possessive form of the word as in their house, their car, their clothes. Personal is therefore possessive and includes the letter 'i'. They're is a contraction. A blend of two words they and are, allowing for the softening of the language and the slur of the words, removing the 'a' from are and adding an accent to blend the two and become they're. The word is used to describe who they are, what they're doing, and how they're behaving in the common gist of conversation. They're my three favorite words to confuse and explain.