Mariah Carey — in the Diana Ross role, naturally — joins vocal forces with Ariana Grande and Jennifer Hudson to sing “Oh Santa!” It’s more like, “Oh snap!”This holy diva trinity goes down as the must-see moment in “Mariah Carey’s Magical Christmas Special,” which started streaming on Apple TV+ on Friday.Over the happy, hand-clapping jam, the three songbirds do more cooing than real belting.