The above post is correct. I can multi-box a Heroic with three toons, a Mage, Priest or Druid, and a Paladin. BUT I had to get Nax+ gear on the Mage before I could do it successfully. The Prot Paladin and the healer just wear heroic gear. I just let the Pally take the brunt of the damage up front to build aggro and then release the Mage on them... with 25 man Nax gear he cooks them.