I'm still uncertain about the third fairy power. I think it has something to do like charging someone to become strong which involves wiping out foes without hurting friends.
So far,there are two Known fairy powers:
Fairy Law: is a Caster Type spell utilized by Makarov and Laxus Dreyar. When activated, a bright light envelops the area and inflicts heavy damage on whoever the Caster perceives from their heart as an enemy, leaving friends and bystanders completely unharmed.
Fairy Glitter:is a Magic that rivals Fairy Law and is a Radiance of merciless light that rejects the presence of any foes.
3 - Not yet shown
So if ever the 3rd great magic would be revealed,who do you think would possess that?