Three Celestial Dragons

The Three Celestial Dragons refers to the three species of heavenly light-based dragons - The Sunlight-, Moonlight-, and Starlight Dragons. The only known living examples of these dragons were Solaris, Lunasa, and Asteri, respectively. These three were sages among the dragons in the "Fairy Chapter" and were the ones that ordered Nero's group be given the dragon eggs.

The Big Three
The Three Celestial Dragons were all siblings. They had never produced eggs despite the fact that they were among the oldest dragons. All three of them were serpent-like Eastern dragons rather than the more prevalent Western dragons in the "Fairy Chapter" world. As they were among the oldest of dragons, almost none of the dragons could remember these three when they were young and those that could further yet couldn't remember where they came from or who their parents were. They were very wise and respected dragons, and they chose the pro-human side of the Dragon Civil War.
 * Solaris, the Sunlight Dragon was the head of the three. His bright red "sunset-colored" scales shone brilliantly in the sunlight.
 * Lunasa, the Moonlight Dragon was the one of Solaris's sisters. Her silvery-blue "moon dust-colored" scales glowed softly in the moonlight.
 * Asteri, the Starlight Dragon was Solaris and Lunasa's sister. Her purple scales "glittered like stars" in the light.

Wise Sages
These three dragons acted as sages among dragonkind. After Nero came to the dragons asking for the power to stop Acnologia, the dragons all denied him and several of the pro-human dragons even suggested killing him due to his ties to Zeref and Acnologia. However, these three stepped in and tested everyone's hearts with their magic. They had decided that since they all had experience in forming human magic already, learning to form Dragon Magic would be impossible. However, they entrusted a dragon egg to each of them. Though some of the dragons still resisted, in the end Nero's group were given the eggs.

Some time later, they also charged Nero and his group with the task of hiding the remaining dragon eggs before Acnologia finished his ascendence to power among the dragons. Whether or not they were killed by Acnologia in the end is unknown, as they had not been seen by human nor dragon ever since.