Thank you for your repliy, but this didn't fix my problem.
I think this is an Arma 3 problem, because if i manage it to get to the editor i see the red square which i can move with my mouse not in the middle of the red cross. The middle of the cross is somewhere on the right but i can't see it. If i start the game in windowed mode everything is fine. The red square is perfectly in the middle of the red cross.

And if i minimize the game to tray and reopen it, it changes the screen to a different one ! (Where it works fine... but this is not the main screen)