Theirs – ‘Theirs’: This third person plural possessive pronoun, used for their + noun, is often used incorrectly. ‘Theirs’ shares a similar grammatical function with other pronouns showing possessiveness, such as ‘mine,’ ‘ours,’ ‘yours,’ ‘his,’ and ‘hers.’ (Both ‘its’ and ‘one’s’ are used quite rarely, and their appropriateness is still a matter of debate.)
In the case that ‘theirs’ appears as the subject of a sentence, the related verb can be in either the singular or plural form, depending on its actual reference.
Meanwhile, the word ‘their’s’ is incorrect and does not exist in English.
However, the word ‘there’s’ is the short from of ‘there is’ or ‘there has (been),’ and is grammatically correct.