The Three Knights are the dragoons, Slust the Red, Fenril the Ice and Dragon Fang Fogel. Powerful warriors of the mortal world of ages past, it was said that a blow from one of the knights could kill a dragon, and that they could even commune with and tame dragons.
The Three Knights live on three Sky Islands. Slust resides on Muspelm, Fenril on Organa, and Fogel on Shiguld. They are revered by all that reside on the Sky Islands and worshiped by the people below.
During the Ogre Battle, as humanity made their last stand on the smalls islands of the Kastolatian Sea, the Three Knights emerged from the heavens and pushed back the demons. They also personally sealed Galf, an underworld general, in the land of Antanjyl. At the war's conclusion, the Three Knights ascended back to their Sky Islands and were made to vow that they would never again help humanity.
Many years later, Rashidi found his way to their Sky Islands and charmed each one. However, the Xenobian Rebellion broke them free of the mind control magic Rashidi had used and the three joined the rebellion, once again fighting alongside humanity. After Rashidi had been defeated they returned to their respective Sky Islands once more.