|Three words, three meanings.|
We'll quickly explain how to tell the difference:
In, at or to a place. Are you talking about a place? Then it's "there" you want to talk about. You can also use "there" as an exclamation. So there!
This is when something belongs to "them". If you're talking about owning something or "having" something, this is the time to think of "their". It's possessive...
Why have an apostrophe? It's a contraction of "they" and "are" because English is a lazy language. If you get it wrong frequently consider pronouncing separately until you can wear your big-boy pants and attempt to use contractions.
These three words are classed as homophones as they are pronounced the same in most dialects.
|Not that type of homophone!|
Words that are written the same are homographs. There, their and they're are written differently and are therefore heterographs. They're not interchangeable and frankly we should stop putting so many homos and heteros in the same post before we get into trouble.