I would have to say that the bordure for the third son would be:
a bordure per pale Argent and compony counter-compony Argent and Azure. Lyon Clerk advised a friend of mine not too long ago that the bordure compony counter-compony would be granted to the son who had the same situation with tinctures as your hypothetical exercise does.

It is also possible that the current Lord Lyon isn't wedded to the Stoddard System of differencing cadet arms.