ORIGINAL: Psyco
As I mentioned above, in order to use the 3rd line you have to pinch-off the line to the carb or close the needle valve so you don't flood the engine.
It is not necessary to pinch-off the line to the carb if you fill the tank via the 3rd line. I don't do it when filling all my inverted four strokes, and I don't recall getting any engine flooded. Fuel likes to take the path of least resistance, so it will flow to the tank.