The screen should be small enough where you don't have to move your head to see different parts of it. A smaller monitor means less eye movement, means faster reaction times.
3 monitor setups are okay as long as the middle monitor shows what a single monitor would have shown you(you can get to all the ui, etc...) In other words, the side monitors are adding peripheral vision.
So ideally, if you are a pro gamer, you want 3 medium sized 120hz monitors, each with their own gpu.