Lone wolf,
I asked the question because I have had a number of traps when the target wasn't the three. I was questioning the use of the word always. I have even managed to be graded a Fair One, because the three and four weren't there. I have also had an OK four wire.
In fact in LSO NATOPs (from memory) it gave a very in depth account of how the roll angle of the fresnel lens could be adjusted to affect hook impact point wrt the target wire but never actually stated what the target wire was.
Just to enhance the debate further I had my first trap on a three wire boat.