AC11 pilot flying reported an NMAC event while turning final approach with another aircraft turning final from opposite base. The other aircraft executed an evasive maneuver to avoid a collision.

Narrative

I was originally given a left downwind visual approach to [Runway] XX at ZZZ that was then changed to a right downwind. I was cleared to land as number 3 following traffic. There is a lake under the final approach course and a hill to the west of the lake; left traffic crosses over the hill on base and turns final behind the hill while right turns both base and final over the lake. I reported that I had aircraft visible over the lake. I could see only the aircraft lights against the dark background of the hill. This was in shadow due to the lighting; I believed this to be the traffic I was following and that number 1 was ahead and not in sight. This proved to be incorrect; the aircraft light I saw was in fact the number one aircraft; a helicopter. I turned final to follow this aircraft; and immediately after turning final; the other aircraft on left approach; a Cessna; appeared turning final above; ahead and to my left. They saw me below and did a go-around at this point.I believe parts of this area may not be easily visible from the Tower. I believe the issues are aircraft simultaneously on left and right approaches; limited visibility due to low sun angle and terrain leaving the final approach course in shadow looking from the east; lack of clarity about the position of the two preceding aircraft in the pattern; probable lack of direct visibility from the Tower; and my failure to clearly state that I could see one aircraft; not both; and to confirm with the Tower whether the aircraft I saw over the lake was number one or number two.