First of all, all spells are situational. You don't cast Cure 1 unless the situation calls for a Cure. Duh. Sounds like you fail to grasp what a situational heal is. Second, the situation doesn't happen anyway. This spell isn't a situational spell, it’s a mechanic spell. Its only purpose is for specific encounters that force a party to stack, not just next to each other, but ON TOP of each other, and eat damage. That is not a good spell design. That is just dumb. The point of a Cure 3 is meant to be a way for the healer to heal up a group, without having to actually be in it. The problem is, it never happens. Nobody stacks like that unless they are on a voip and are trying to save the healer a bit of effort.
There is no damage right now that happens where a Cure 3 would be useful, as Keilei posted. It would have to do a crap ton of damage twice in under 9 seconds and everybody would need to be SUPER stacked for it. Plus it would have to be damage that only happened for some of the players but not all, because Cure 3 is a range AOE cure, so the WHM shouldn't be in the stack. This does not happen in the game naturally. If any battle needs it, it was designed to make the spell forced to be used, rather than the natural situation where an emergency AOE heal needed to be used. Plus what’s the point of Overcure if the only time damage like this happens is scripted. You can't guarantee a proc at that point, 99% of the time Overcure is useless. It is a poorly designed spell and trait. Fact.
Right now it needs a slight range buff. Then it becomes that situational AOE heal it’s meant to be, the one that you do use when a lot of burst damage happens and a group of players are next to each other, rather than the “you screaming in voip, MY OVERCURE IS UP AND YOU GUYS ALL JUST HAPPENED TO BE DAMAGED STACK TOGETHER SO I CAN USE THIS!” spell it currently is. It and it's trait need to be looked at. A 4y to 6, would probably make it perfect. Even when DPS looks stacked, Cure 3 often magically seems to not hit anyone but the guy you targeted. Maybe even 5 might do the trick.
One more edit: I see where the spell has its uses right now. The problem is more often then not, the WHM is in the stack as well and there is no Overcure. Which defeats the purpose of its range. So when I say poorly designed spell I mean the whole thing, not just some parts. DPS doesn't usually stack close enough for it to hit everyone, likewise when people do super stack, the WHM is usually included, likewise Overcure probably isn't up. The chance that the people you're trying to cure are SUPER STACKED + Away from you + Overcure being up... Its magically rare. So rare, that the WHM probably wont even think to use Cure 3, knowing most likely half of who he is trying to heal wont get healed or he forgot he had the spell. Don't forget when damage like that happens, all the DPS is usually trying to run away, so they're all spread out either running away or coming back. Once again a tiny range increase would open this spell up to have more uses than that special moment when a group took a ton of damage, more is on the way, you're 20-30 y away, and they're all stacked and staying still, and maybe you casted a Cure 2 that just happen to proc a Overcure. Likewise DPS shouldn't be taking that type of damage as group, it means they're doing something wrong. So the whole thing is just really rare and not natural and a tiny tiny tiny range increase would make this be the spell you're trying to defend, rather than the spell it currently is.
Cheers.