Well, at first I thought I agreed with you all, but as I wrote my interpretation, realized I disagree to a certain degree.
If you were looking at a photograph of a group of people lined and rowed up, and you said, "Who is that man on the second row, third from the end?" Well, you would count the end as "first," the second person there as "second," and the third person from the end as the antepenultimate person--or third from last. I wouldn't count the third from last person as being the fourth. It makes sense to me that we go from last, to next to last, to third from last because as we count things, we naturally progress from one number to the next. If you're counting backwards from a point, last becomes first, next to last becomes second, and third from last is third.
In other words, we have first, second, and third in line; when we count backwards from the end of the line on up toward the beginning, we still maintain first, second, third, etc.
Therefore, third from the end (or third from last) would clearly be antepenultimate in my way of reasoning.
Third from last is third (antepenultimate), not fourth.
