All Delta 767-300s (domestic configuration, ER and non-ER) have 2 doors on each side... one in the front and one in the back, as well as 2 overwing exits. See this photo for an example. There are 80 DL 767-300s configured this way.

However, there are 7 Delta 767-300ER's that have 3 doors on each side (and only one overwing exit) : N1501P, N152DL, N153DL, N154DL, N155DL, N156DL and N394DL (tail #1521), but these are all in international configuration. Even on these I don't think the second door is used for boarding, but I may be wrong.