i never played 3boxing but done some dualbox,
but if i where to 3box i wold like to do paly tank and some nukers behind me maby 2 priest for the vampiric embrace no need for healing here:P you get heal + shitload of dmg;P
but if you change priest whit shaman and use your healing totem it wold be the same:P
this is just my ide. the shamans give you realy good totems to buff your pala