Keep it simple!
I found using an extra (third) line for fueling be unessesary, only one more thing that can start to leak and you need to plug it or clamp it when the engine is running.

Simply separate the carburator fuel line with something. I use the L shaped, plastic fuel tube that comes with the Great Planes fuel tanks. You can also use a piece of brass tubing.

To refuel, separate the fuel line and fill up the tank, then reattach the fuel line and start the engine.
Simple and foolproof!