Input (grade 13): While technically the term is limited to the third largest party or third oldest party, it is common, though innumerate, shorthand for any smaller party.
Output (grade 12): While the term should be used only when discussing the third largest party, it is often used to describe any smaller party. For example, in the United Kingdom a third party is a national political party other than the Conservative Party and Labor Party that has a presence in the House of Commons.