Is theirs a pronoun

Yes, the Word 'theirs' is a possessive pronoun, a Word that takes the place of a noun for something that belongs to two or more people or things.

Example: Mark and Mary live on this street. The house on the corner is theirs.

The possessive adjectives are placed before a noun to describe that noun as belonging to two or more people or things.

Example: The Martins live on this street. Theirhouse is on the corner.