By itself, the idea to give bonus effects to Skulls based on how many enemies you can hit at once with them is interesting.
Getting 3 targets at once with one ET use isn’t that difficult. Let’s list the possibilities:
-Tomb (mostly against bosses, but could sometimes work during rushing)
-Cdepths (as long as you have small eggs in the vicinity)
-Shatters (mostly 3rd boss)
-Godlands (extremely situational, but it’s doable)
-LoD (except Ivory)
-Shaitan, ironically enough (especially if they add more status effects)
-Cnidarian Reef (extremely easy)
-Oryx Castle (also O1 minion-spawning phases)
-Wine Cellar (both the rush and the beginning of the fight)
-UDL (mostly in case you get Slowed by Vampires)
-Manor (might fuck up your Confuse controls though, and Bleeding isn’t that dangerous)
-OT (ganked by ponies? skull the seahorses)
-Sewers
-Most realm events (especially useful in case of Urgle)
and probably more but I can’t be bothered.
As you can see, it’s not exactly the most situational, niche usage ever.
The main problems with this change are, in my opinion:
-much harder to use as a group puri than a selfish puri
-unreliable, not as consistent at saving your ass from sudden Paralyze instakill shenanigans and the like (especially against enemies with temporary Invulnerability and enemies with erratic movement patterns)
-not a good fit for ET: might be better to give it an effect that puts even more emphasis on damage and reserve the puri effect for a more defensive Skull.