The definition of third class is something or someone that is considered lower in class or rank than second class.(adjective)
An example of a third class train ticket is one where you get only a tiny little seat and no access to a restaurant or snacks, unlike people in first class with whole railroad cars to themselves or people in second class with a little cubby to themselves.
See third-class in Webster's New World College Dictionary
See third-class in American Heritage Dictionary 4
Learn more about third-class