Since no one actually answered your question regarding that third fuel line coming from the tank, that is your fill line. You can fill the tank one of two ways: 1. remove the fuel line that goes from the tank to the carb or needle valve (if it is a rear mounted needle valve) at the point where it goes to the carb or needle valve. You connect your fuel supply to that point. Disconnect your pressure line from the muffler and point it away from you (as suggested.. fuel in the eye hurts!). Begin fueling until fuel runs clear out of the pressure line. The tank is full. 2. Leave all fuel lines where they are. On the third line, you will put a cap in that line, a plug of some sort, to prevent fuel from coming out from pressure when normal operating. You remove that plug from the third fuel line and connect that to your fuel supply and, again, disconnect the muffler pressure line and point it away from you, and pump fuel into the tank. When it runs clear from the muffler, the tank is full.

In case number 1, disconnect the fuel supply from the tube and reconnect the tube to your carb or needle valve and reconnect the muffler pressure tube to the muffler. In case number 2, disconnect the fuel line from your supply and re-plug that line so fuel does not run out of it. Reconnect the muffler pressure line to the muffler.

I made a crude drawing of what you would expect to see. Hope it helps.

Your're ready to fly.

DS