the answer is "They"
The difference in these three:
THEIR is used for people if something belongs to them. As a pronoun you can say e.g ITS THEIR HOUSE,
THERE can be used when you are telling where any noun is e.g YOUR CAT'S OVER THERE.
THEY'RE is a contraction - actually THEY ARE. e.g THEY'RE OUT OF CITY FOR A FEW DAYS.
I hope this will help those who are confused