Three wire parallel transmission line (TDR)
You calling one of the lines &quot;common ground&quot; doesn't matter, it carries just as much info as the other two.
I'm assuming &quot;really close&quot; very much means these lines aren't independent; the moment you change the potential field of one, you also change the field for the others. There's no such things as &quot;I feed a pulse into one, but not into the other two&quot;, that's literally not how transmission lines work; you've got a field between your conductors, in which the energy is transported, and in a very much &quot;action=reaction&quot; way, you won't be able to tell whether one is ground and the other one is fed without a reference:
Maxwell says your changed potential on one line leads to a current, which leads to a magnetic field around that line; the other lines are in that field, and ampere's law kicks in and there's a current induced; that leads to …
It's transmission line theory all the way.
Now, your scenario really sounds complex enough to say: The only realistic way to answer this is through FEM EM simulation.