I'm not too sure, I only use two screens.
I'm only using a single screen for DCS though, I don't spread it out so perhaps if you use multi-screens for DCS, limit it to a single screen and test it out.
I can't say it's DCS that's the problem at the moment if you are the only person reporting the fault and I can't reproduce it myself.
Do you have any mapping software running in the background? (xpadder, pinnacle, keyboard software, etc...)
So try the one screen and see how it goes. By the way, is your graphics card AMD or Nvidia?