I normally setup the 3rd line with a clunk in the same manner as the fuel pick-up line. But if you don't have the materials to make a second clunk line you can make a pick-up setup that works almost as well. Feel the tubing for the 3rd line through the stopper so that it's bent towards the bottom of the tank. Before it's placed in the tank put a small piece of fuel tubing on it so that it just barely reaches the bottom of the tank. Then when it's time to de-fuel the plane simply tilt the tail of the plane upwards and suck the fuel out. With this setup you'll be able to get almost all of the fuel out of the tank.

Ken