If you insist on making a distinction, the ambiguity can be removed by referring to the "third from the last" for the standard/intuitive meaning and the "third away from the last" for the hair-splitting one.

I prefer Tsuwm's Law: if it's awkward, don't use it. (or was that Safire's Law?)

(And now, sports fans, can you think of a word with the letters "-wkw-" in it somewhere, in exactly that order?)