The third principle is less related to the first two, and is more about the evolution of defensive measures. Even if sticking close to fellow soldiers in a formation really helps in surviving, by being protected by the shield of others or simply by having an incentive NOT to rout, it is not enough to maximize your chances of survival on a battlefield.
The very first protective measure a soldier needs to take is to protect the head. Archaeological evidence from the Bronze Age to the Middle Ages shows that the head was the target of choice in war and that it would kill much more quickly than any other wound, other than a stab to the heart. It is safe to assume that any soldier would make sure that they have a proper helmet. T
he shield is the next defensive measure in necessity, but one thing can make shields useless and that is armor. We indeed see that with the development of full plate armor in the late Middle Ages, that shields progressively disappeared on the battlefield, making way to the widespread use of two handed weapons such as halberds, pollaxes and greatswords. The change from using big shields like the Romans to using no shields at all combined with full plate armor is a long story spanning over 1000 years of technological progress.
One thing is for sure, armor is not like in movies where weapons just pierce them, or like in games where it simply adds more “HP” to the wearer. Armor works, armor deflects harm.