When to use Cure 3 or not

[Lyrinn]

If it means anything, when Overcure procs, Cure 3 has a slightly higher Potency/MP than Cure 2.

Last I checked, that was only the case if you hit 2+ people with the Cure III. Otherwise, it's like using Cure II without a Freecure proc and that burns your MP quickly. There aren't many times I can think of where I've needed to use Cure II right before a stack heal and also magically get an Overcure proc.