What is the name of a variable resistor with 3 terminals?

It's called a 'variable resistor'. However, it can function as a 'rheostat' (to control current) when two terminals are connected, or as a 'potentiometer' (to control voltage) when three terminals are connected. The terms, 'rheostat' and 'potentiometer' do not describe the device itself, but how it is being used.