The Holy Trinity is not required at all. Any strategy can be devised by any three characters. Each class has 3 specialties or "specs" that all play slightly differently.
Rogue Assassin has the Fast Attacking Knives good for steady and balanced DPS with a Knife Throw ability that hits everything in a line before him, adding a decent amount of AOE.
Rogue Gunner has slower, high range/high damage shots that when aimed, can tear down larger creatures from a distance, as well as Spawners. Their special is an delayed explosive shot that again when aimed can destroy a larger creature and damage nearby creature on detonation.
Rogue Archer is an advanced class to play, as you can fire individual arrows that are slower than the knives and just as damaging, or Charge your shot for a great amount of damage that pierces through enemies on it's path, always Critting**. Their special ability is laying caltrops down that slow and damage enemies while jumping behind them to disengage the enemy.
I could see a team of an Archer and two Gunners hammering at their foes behind a floor full of caltrops, taking out any large enemies with explosive shots. Healing can be done with a Healing Supplies ability which is a potion you can get as a rogue.
There's a lot of variability, and while I can say that some things won't work as well as others, anyone with enough skill can make up a strategy that will work out with any class.