It's all tactics, minding your surroundings and a whole lot more of team coordination.
We did it with 3 melees on HM died once on each boss including bonus. Guardian (Tank - half rakata half columi), sentinel(dps - columi), scoundrel(dps - rakata) and Sage (healer - columi). When doing sentinel droid with 3 melees, you will have to fight all around the room, with tank moving from side to side and front to center to back, everyone needs to know where they are going.