They were used in Ducted Fans a lot to give better fuel levels for the very large bore carbies.
what if the header tank has two vent lines, what can you do to the 3rd to make it work?
Plug it up. In aircraft today you should not need a header tank, I can't see where you would need one. Set the fuel tank and fuel lines up properly and you would never need to use one.
Cheers