Hi Viz
(Stop that giggling down the back!...)
Yes, "theirs", "hers", "ours" and "yours" are all allowed. They're not plurals or verb forms, but alternative forms of possessive pronouns, required in some sentence constructions. (Do pay attention; this will be on the exam.)
We say, "I visited their house," but, "The house I visited is theirs." If we weren't using a pronoun, we would use the same word or phrase in both cases: "I visited the Smiths' house." "The house I visited is the Smiths'." It's just one of those quirks of the English language.