They wore heavy metal armor and carried large shields, their usual position being the third battle line. They were equipped with spears and were elite soldiers among the legion …
In most battles triarii were not used because the lighter troops usually defeated the enemy before the triarii were committed to the battle. They were meant to be used as a decisive force in the battle, thus prompting an old Roman saying:
Res ad triarios venit. It comes down to the triarii.
U.S. Army War College
Motto Latin Prudens futuri
Motto English Wisdom and strength for the future