1.Their is a possessive determiner. When using their, you indicate that something belongs to them:
Example :
Their car has broken down.
Their dinner party was a big hit.
2. They’re is a contraction of they are, very similar to you’re from tip number three:
Example :
If they’re not coming, I’m leaving early. (If they are not coming, I’m leaving early.)
They’re so happy to have been invited! (They are so happy to have been invited!)
3. There is an adverb indicating a location of something, whether specific or abstract:
Example :
Your keys are over there, on the table.
When we got there, the place was already closed.