Well, im not native english, but afaik one says "i have 100 dollars" to indicate that he has 100 1-dollar-units (um...sounds dull...). Whereas "i have 100 dollar" just means the same amount of money in any denomination (?).
So "100 trias" are just 100 of those little triangular thingies,
and "100 tria" is worth 100 trias, but can consist of hexas and/or octas.
Hope you know what i mean...
just correct me if i am wrong