Not to argue with Alex over in W & F, but I think *this is the funniest discussion we've had in a long time! Welcome to you, birdfeed, by the way.
Here's what Atomica gives for antepenultimate: Coming before the next to the last in a series. That seems pretty clear to me. (It went to Google for pre-, and there wasn't anything clearer that I saw than what's already here.) My opinion is that if you accept 'next to the last' as being what we normally think of it as--say the letter Y in the alphabet, that when you say third from the last, you cannot THEN decide that Z is not, after all, the last.
Possibly the word 'the' is adding to the confusion: next to last (why do I want to hyphenate that?), third to last, etc., seem a little clearer.