What should my third materia be?

[VanilleFang]

I use VIT in that case. Saved my life more than once. Moro hears about it all the time whenever I survive something with 84 HP.

VIT is also an option. Not going to lie. I didn't think about putting it on left side equipment. I usually only save it for right side.

Though again VIT is like PIE. Anything above what you actually need is sort of a waste, but it's still an option regardless.