May I add, I have a 3Monitor setup 19"|24"|19" working without the need for softth in DCS 1.5.3. Full Resolution is 4480x1080. Did not have to make leftmost the main one. Primary monitor is still center. That is on 1Screen/1Camera .lua config.
Though I tried setting up softth 2.08(x64) hoping to solve a different issue, it was simply not recognized by the game at all (no "softth" splash on startup or anything). I did drop the d3d9.dll into ...DCSWorld\bin