I have been 3 boxing as well with:

Cleric ( Sentinel / Justicar / Druid )
Rogues x 2 ( Marksman / Ranger / Riftstalker )

Only level 10 due to sick kids, work busy, etc ...

However, loving it so far and with the above combo killing things is pretty easy.
Need to set up some priority macros tonight to invoke cooldown abilities before spamming the instants.

A friend of mine is single boxing a melee based rogue.