I don't understand your confusion. Haven't you played FFX-2?
They were the human heroines who saved Spira from destruction. But then their wold got swallowed up by darkness. Rather than becoming Heartless, they were able to become fairies instead.

Then Maleficent hired them as spies "until Maleficent left them to fend for themselves during the massive battle. Donald's lie causes the trio to work for Leon who was said to have more treasure than their former employer. Eventually, the three learn the truth and want payback from Sora. However, learning of everything Sora and friends went through, the fairies decide to let Sora and gang off the hook by giving Sora a keychain as a gift."