I don't think there will ever be a situation where a single mob is resistant to Ice, Fire AND Thunder all at once. There may be an encounter where certain mobs are immune/resistant to one at most but never all three.

The elemental wheel is a nice touch, however we have no idea what element most enemies are aligned with. This is why we need the Libra/Scan/Bestiary in the game. So that players may be properly informed and can use that knowledge to play skillfully.

Edit: Also the current element wheel, or set of triangles, is fine. It's just different and not obvious so most people hate it.

Fire > Ice > Wind > Fire
Earth > Thunder > Water > Earth

Pretty simple actually, jut a matter of remembering it.