Well with 3 characters, you get 3 mercs.
so you have a few choices.
Either, go with classes that can do things the mercs cannot do, or just go full on dps.
Standard 1
1 tank (paladin, shadowknight, warrior)
1 CC (Bard, Enchanter)
1 Heal (Cleric, Shaman, Druid)
3 x dps mercs
Standard 2
1 tank...