Which member of the Triple Alliance fought on the side of the Triple Entente?

The answer here is Italy. Italy was a country that first fought for the Entente forces (which included Germany) in the First World War. During the course of WW1, the Italians switched sides and started fighting for the Triple Alliance (Britain, Russia, and France). This meant that the forces of the Triple Alliance eventually had another source of soldiers which were fighting for their cause.