For the Necromant and the Overlord, the closest analogues will actually be the Diablo II builds for the Necromancer known as the artillery necro and the overlord. So, one will be a blasting caster with a little melee muscle to keep the peons away, while the other will be designed to have a large number of companions, at least relative to D&D, and not a lot of spellcasting might.
The Vivisector I'm uncertain of myself, although the closest class in existence to what he will turn out like is almost certainly the Ritual Warrior and his abilities.
BTW, I'm almost certain the Necromant will get 2 majors, 2 minors, and 1 banned, the Vivisector 3 minors, 2 banned, and the Overlord 1 Major and 2 minors and 2 banned.