The third line, when used, is a filler. That line would be easily accessible and negate having to remove the fuel line from the carburetor when filling the tank. Usually the holes in stopper assembly do not extend completely through the stopper and are air tight. So, if you don't use the line, there is no air leakage from the hole. When used, the small piping is pushed through the holes, thereby opening them.

If you have easy access to the carburetor, its easy to remove the fuel line, i.e., the one with the klunk on the other end, for filling. If access is a little difficult, use the third line as a filler, but you will need to plug the end after filling to prevent air intake.

DaveB