Once you draw three Cards of the same element, all the Heroes of that element activate and deal a three-sword attack, hitting all monsters in that specific room. As an example, if there are four monsters in the room, the activated Heroes will attack all of them.
You really want three Cards of the same element, since you will deal more damage (if the Heroes of that element are particularly violent ones, armed with Melee traits, for instance) and thus emerge victorious from the Dungeon more quickly.
Good luck! As an extra bonus, try to reach three cards and Super 21 :)