Sacrifice a creature: Draw cards equal to the sacrificed creature's power, then discard three cards.
If you don't have three cards in hand when instructed to discard three cards, you discard your hand.
Use the sacrificed creature's power as it last existed on the battlefield to determine how many cards you draw.
You draw and discard cards all while Greater Good's ability is resolving. Nothing can happen between the two, and no player may choose to take actions.