On a trainer, since the engine is out in the open, I would not use a filler valve. Just disconnect the carb line and fill through it. If you do use a filler valve you don't need to use the 3rd line in the tank. If it's not open, don't open it. If it is open, you'll need to seal it. I made a 'very rough' drawing of a 2 line system with filler valve.