추공 (Chugong)
It doesnt make sense that 3 people couldnt cover their surrounding with eyes. Humans have Field of View around 120degree. So if 3 of them stood back to back and watched, they should be able to cover all 360 degrees. Although that Field of View might not be perfect 120degrees, but stone statues are usually big. So they should not be able to slip through those mini 1-2 degrees blind spots.
I think we can say it’s because of the blinking frequency? If there’s 4 people, then even if one person blinks, the person to his immediate left and right may not be blinking at that time, thus covering for him. But if there’s only 3 people, there would be more dark spots where if one person blinks, his immediate left and rights field of vision may not have that dark spots under their supervision? (Since the angles at doctor who also moved with that mechanism?)