Any progress on this? It's a right pain in the ass on my triple monitor setup. I constantly click besides the window, even when I'm not using a UI, forcing Arma III to minimize.

This happens to me WHILE I'm in fullscreen mode, not windowed. My cursor moves to my other monitors simply by moving my character around.

For now I'm working around the issue by using DisplayFusion's mouse locking functionality. But this really should be a default option in the game.