The most popular assists for Magneto and Dormammu by far are EM Disruptor and Dark Hole, respectively. They are definitely the best assists for neutral on those characters. I know some teams use Hyper Grav for Magneto, which is worse in neutral but has some combo utility, but I don't think Mag/Dorm/Ammy would really benefit from doing that. I've seen people use Liberation or Purification on Dorm for combo video stuff, but as far as real matches its Dark Hole across the board.
I don't know anyone who plays that specific team, but the major Mag/Dorm players (FChamp, Ranmasama) all ran Disruptor and Dark Hole. Zak Bennet played Firebrand/Dorm/Ammy and he used Dark Hole assist there.