If you play 3 (or more) different cards in your turn that cost 1 or more (i.e. not starter cards or weaknesses), you add 3 power to your total power this turn. By different cards it means cards with different names (for example, Man of Steel, Fastest Man Alive and Batmobile) rather than different types (equipment, super power and hero).
This makes Green Lantern powerful later in the game, but not in the early stages.
This makes Green Lantern powerful later in the game, but not in the early stages.