What are the three classes of levers?

A class three lever is where the load is one side of the force and the fulcrum is on the other side of the force. An example is a pair of tweezers. Another example is the secondary lever in a pair of fingernail clips, i.e. the part that closes on the fingernail. (The first lever in a pair of fingernail clips, i.e. the part that you press on, is an example of a second class lever.)