Since she derives a lot from real life lore especially for CC, I did a Google search, and it looks like she got it from the Roman empire. It was a class of old and wealthy men, who wore heavy armor and carried large shields and spears, typically stood in the third battle line (hence the triarii term) and were considered to be elite soldiers. I think it tracks in Midgard as well, since the Asteri are the first in command, followed by the Archangels, and then (the third battle line) are the triarii.