There is no reason to use Tri-disaster for damage. Ever. Long cast time, garbage potency, eats a ton of MP. For AoE if you don't care about MP, Swiftcast Shadow Flare, Miasma2, spam Blizzard 2.

Miasma 2 is net 70 potency/targer, Blizzard 2 is 50 potency/target. You need only look at the tooltips yourself to see what to do.