it depends on the machine you want to control with it.
basically, the joke is designed for a 1-engine piston. here, the black (left) lever is the "throttle", i.e. the thrust of the engine, the blue (middle) lever is for the blade position of the propeller (depending on the model, if available) and the red (right) lever determines the fuel-air mixture (low fuel to high fuel).

you can of course freely assign the levers in almost any flight simulator (P3D, FSX, XPlane, FG,...) and of course proceed in a way that suits you. the above is the default setting if you simulate a plane where these 3 functions (throttle,propeller-pitch,mixture) are present, and even there you can optionally (not very realistically) make a completely different setting.