A third-class lever has the input force in between the output force and the fulcrum. You can calculate the mechanical advantage of a third-class lever the same you would any class of lever. Third class levers will always have a mechanical advantage less than 1. Usually the purpose of third-class levers is to transform work and increase distance, and therefore speed, of the output work.
Examples of third-class levers in everyday life are hammers, tennis rackets, and fishing rods.