Ursula-changing into and/or controlling creatures of the sea (signature magic)
Maleficent-changing into and/or controlling creatures of the air (signature magic)
Cruella-changing into and/or controlling creatures of the land???? (Signature magic).
maybe Cruella is nothing special on her own, but when the three of them are together, they can enact a Super Spell (or something?)
Regina can turn herself into rats, but perhaps it is not her signature spell (whereas fireballs might be).
Perhaps Regina was originally part of the trio, but because metamorphasis was not her signature spell, and because ursula didnt trust her after Regina impersonated her, Regina left the trio (or was kicked out) and Cruella was her replacement?