No no no, the way this works is that: You use the ability. 3 creatures damaged your health bar last turn, so this behemoth gains 3|3. Next turn, 9 fireflies damage you. You use the ability, you gain 8|8.
Now reading this, i realize that it would make more sense to only use this ability 3 times, max of +5|+5. Thank you for pointing that out. Facepalm.