Ok, I had to drop off something at the post office, so on my way out I looked very carefully at the intersection.
BL, you are right; there is no arrow or overhead lane sign for the lane in question.
However.... Lane 1 and 2 (left-most 2 lanes exiting San Souci onto 235 NB) have arrows painted in the lanes clearly identifying them as left turn. Lane 3 has no arrow.
Between lanes 1 and 2 is a dotted line. Between lane 2 and 3
is a solid line. Between lane 3 and 4 (rightmost turn-right-only onto 235 SB)
is a solid line.
Lanes 1 and 2 are clearly a pair of left turn lanes with arrows above and below. Lane 3 is separate and distinct from those two lanes, separated by a solid line, and is not intended for a left turn, as is lane 4, separate and distinct from the other lanes as a right only.
You are the only person I know that believes and uses lane 3 as a left turn lane, and I can fully understand why you get cut off. I have never seen anyone else make a left from lane 3. I strongly suggest you don't turn left from lane 3 anymore.
That said.... yes, an arrow or overhead sign would remove any doubt. And while they are at it, they can do the same for Town Creek Drive exiting onto 235. It's marked pretty much the same way, and people going straight conflict with those turning.
Look at the overhead view. No question in my mind.
California, St Mary's, Maryland - Google Maps