You should never write 'theirs' with an apostrophe. 'Their's' is incorrect in standard English, so you cannot use it.

Especially with those possessive pronouns, like hers and theirs that end in 'S' already, you might instinctively add the apostrophe. But remember that it is wrong.

The third-person plural possessive pronoun in the English language is 'theirs'. It can be used to refer to something or someone belonging to or associated with 'others'.

'Theirs' can be used instead of 'his or hers' to show that something belongs to a person without mentioning the gender of those people.