The plural of Tria is definetly Tria! Or was it Trias? Or both?

In fact I think it's both, but in different meanings.
The plural of Tria is Trias, if you are talking about the coins, but Tria, if you are talking about currency or worth.
Remember the coins Tria, Hexa (10 Tria), Octa (50 Tria) and Circle (250 Tria).
So, if you are having one Hexa, you are owning 10 Tria until you change them at the "Bank of Hydlaa" to 10 single Tria coins. Then you are owning 10 Trias with a worth of 10 Tria

Since you are able to change one Octa and a Circle, which are having a worth of 300 Tria, to coins, which would be 300 Trias, you are allowed to say either Tria or Trias.
* Quantus likes to count Circels and not Tria