Note: 'Theirs' is the third person plural possessive pronoun.
1.
You use ~ to indicate that something belongs or relates to the group of people, animals, or things that you are talking about.
There was a big group of a dozen people at the table next to ~...
It would cost about ?3000 to install a new heating system in a flat such as ~...
PRON
2.
You use ~ instead of ‘his or hers’ to indicate that something belongs or relates to a person without saying whether that person is a man or a woman. Some people think this use is incorrect.
He would leave the trailer unlocked. If there was something inside someone wanted, it would be ~ for the taking...
PRON