This game keeps teasing me with little mentions of “three suns.” The first time I noticed it was in the last act of the Fontaine Archon Quest, when Furina said something like, “It’s not like there could be three suns.” Okay, likely story.
It just came up again, in the description for Absolution:
“Leticia, have you ever imagined a world—” “One with three times the sunshine and boundless fertile lands,“
Considering there were also three moons, I don’t really think it’s that far of a stretch to say there might have also been three suns.
In Moonlit Ballad of the Night, Alice receives a letter that says “Aralos / Solnari / Cannon.” These are very close to the names we knew for the Moon Sisters, and given the fact that everything in Nod-Krai has to do with the moons, it’s probably just referring to them. But I suppose there is also a possibility that these names are slightly different because they’re actually the names of the three suns. Especially because sol from Solnari means sun.