Not so sure I understand your set up right.
I also use 3 lines.
#1. with clunker is the pickup line goes to engine fuel intake.
#2. Filler line which is connected to the muffler to pressurize the tank. I disconect it from mufler for refuling.
#3. The third line is strictly an overflow tube that blows out extra fuel when the tank is filled up, I plug it with a screw after refuling. This prevents me from flooding the engine when fueling.

Worth noting that #2&3 are tubes which are bent up towards the top of the tank.

You can drain the tank by using the fuel line attached to the engine but that may be cumbersom.
If u turn the model upside down u could use any of #2 or #3 to drain the fuel with.

Worth noting all of the fuel lines attached to the engine or tank should be secured to their tubes with clips.

Hope I didnt confuse u.

Rafeek