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.