In some of the new lore the power that infused Diana is described as being of the "Silver Moon", which is described as an ill omen for the Rakkoran in Panth's lore, so that's one of the two. I dunno what the third might be like if it exists, but there's probably a "main moon" so to speak and the Silver Moon is a smaller moon that orbits slower than the main moon.
I feel like the overlapping reality thing is what Mount Targon was meant to do for Runeterra from the start; be the connecting point for realities that intersect with it, almost like the front door of Runeterra in a dimensional sense. I imagine Runeterra is of some importance on a universal scale to warrant beings from other realities empowering beings from Runeterra, but as to why who can really say. Perhaps it links in with the intersecting dimensions and such and if Runeterra were to fall all other worlds linked to it would be at risk, so forces from those worlds use what power they can to aid the denizens of Runeterra and protect them from threats from within and from...outside.
In line with that the Aspects of Mount Targon, the Ascended of Shurima and the celestial beings like Bard and Soraka could all be from different planes intersecting with Runeterra who operate in ways that their home plane allows; Bard has free agency to go where he wants, Soraka was bound to her grove but could help mortals, the the powers that created the Aspects and Ascended were like care packages being sent through to Runeterra to help its denizens. I'd be willing to bet the folks from Bard's plane are the ones more in control of this situation and kinda coordinate stuff, as they're the ones who have a special agent to deal with issues when shit really hits the fan. Of course this is assuming these different dimensions that have helped Runeterra in the past actually work together in some respect or did so in the past.
I feel like I went off there towards the end but maybe it made some sense.
