If the opponent has more Level 3 or lower monsters on the field than you have empty Monster Card Zones after you Tribute this card (the Zone it occupied is now empty), you choose which ones to gain control of and the rest are destroyed. If you choose to destroy "Griggle" or "Ameba" in this manner, their effects do not activate since they never came to your side of the field.
You cannot gain control of face-down monsters with this card’s effect, so the opponent may chain "Book of Moon" selecting a Level 3 or lower monster and they will keep control of it.
You maintain control of the monsters until they are removed from the field.