While i don't have any experience 3 boxing, or dual boxing for the matter, i think i might be able to offer some insight.
for tank, i think SK would be good idea. They can FD themselves or others, so i would make a macro that targets healer and FDs them in case of emergencies. and on easier targets they do great damage too.
For dps i would go warlock, god they do crazy group damage these days. and the porting you mentioned.
For healer i would go defiler, the group ward is a god send and you can pre group ward, so if you mash a warlock group nuke too soon, everyone in the group has a fat ward on them while your SK gets agro back.
Just an idea, i'm sure other people have other good ideas of trios. The way i've outlined it, it should be pretty easy to manage. Group ward, pull with SK, drop a couple taunts, NUKE NUKE NUKE NUKE... death to mobs. you prolly kill most mobs (other than named) before your group ward falls off. Also, just leave the group ward queued again after it's cast once. Should be able to leave healer alone and focus on managing taunts vs dps.