If you want one build for all of T3 you could look at this.
http://is.gd/YHx1su
It's got Despoil the Soul for Arbanus and Gyas fights, and No Rest for the Dead so you can play DtS in puzzle phase of Thoth Amon, and get quick spawns of pets again when you wish to do that. And after dying. Almost no point of combat ressing a Necromancer without No Rest for the Dead, he will spend rest of the fight on spawning pets anyway...
The rest is pretty standard. Ancient Soul to lengthen life of your Parasitic Soul pets. When conserving mana, use spellweaving buffs for getting pets and boosting base spell damage. Death God is awesome indeed but it's very hard to keep up mana in some of the fights.