The problem gets solved after you restart your PC.
If you run Warcraft 3 on a modern computer with 2 or more monitors, the brightness will turn very high for your monitors that are not running the game.
After you exit Warcraft 3, the brightness will still remain very high for your desktop on all monitors.
As I was saying, if you restart the PC, the problem will be gone. Don’t bother with trying to modify the brightness for your monitors, is too much work.
This problem appears when you run a video on your second or third monitor. Or other programs that require your video card.