Evocation Reflex Save could work. Most spells that have this is evocation.
No, many monster abilities (most notably dragon breath) work this way, too.
Please, don't. That way, some players will be confused about how many dices would they roll. If it hits 5 enemies, two of them succeded, two of them failed and one critically failed. How many dices do they roll? Three for the first two, six for the other two who failed and twelve for one of them?
Seems over complicated. I like the idea of rolling once and halving/doubling, which is the way we do with 1e and it's easier because it's basic math.
EDIT: grammar.
Its not hard. If every target crit succeeds you don't roll damage. If one or more targets succeed roll 3d6 and note the result, if one or more targets failed roll another 3d6 and note the result, finally, if any targets critically fail roll another 6d6 and note the result. Apply the appropriate total result to the appropriate targets. Boom done.
Alternatively:
A burst of fire explodes. Creatures in the area must attempt a Reflex save with the following possible results:
Critical Success The creature is unaffected by the spell.
Success The creature takes 6d6 ÷ 2 fire damage from the spell.
Failure The creature takes 6d6 fire damage from the spell.
Critical Failure The creature takes 6d6 × 2 fire damage from the spell.
Heightened (+1) The damage taken by the creature increases by 2d6 (before being modified by success or critical failure).
You're supposed to be rolling multiple sets of dice for most (if not all) criticals anyway (see below). Though most players ignore this rule I imagine.
Critical Hit Damage
When you double the damage on a critical success with a Strike, or with any other action or activity that multiplies damage, use the following rules to determine which values you multiply.
• Roll double the usual number of damage dice for your weapon or unarmed attack.
...
There are three more points but none of them mentiones spells specifically, so they fall under 'or with any other action or activity that multiplies damage' and use the first bullet's rule of doubling the actual dice rolled, not the result obtained. Also per pg 197 Spell Attacks are considered Unarmed (and so fall under the first bullet anyway).
So my quoted procedure above is actually the only legal way to adjudicate a Fireball. Directly multiplying or dividing the 6d6 result is much faster, but also not the procedure indicated by the RAW. So in the case of a typical Fireball, you have to roll damage up to three times, just as I described above (and again below).
If one or more targets succeed [against a standard fireball] roll 3d6 and note the result, if one or more targets failed roll another 3d6 and note the result, finally, if any targets critically fail roll another 6d6 and note the result. Apply the appropriate total result to the appropriate targets.