
I don't really know since I just saw some spoilers but this is the explanation you need that I also saw from the people who've read the novel
If Nyne gets too heartbroken, he can vanish completely. At the end, Nyne becomes transparent. Since he couldn't handle everything that was happening. He started wandering around. Gwen kept looking for him but struggled because Nyne's form was barely visible anymore.
Eventually, Nyne shows up again (Physically) at the end during the ritual. When gwen and amon came back to trastasa, I think they brought nyne too but the people there can't see him at all.
The ending feels kind of open. The last POV is from the servant, so we don't really get a clear picture of Nyne's feelings at that time. But after everything, it seems that Gwen, Amon, and Nyne return to Trastasa and live together peacefully. So, it might be hinting at a 3P ending.
Guess that's why people said it was a 3p
I saw spoilers from the novel and apparently in the ending nain didn't have to choose between gwen and amon so it's a 3P!!!! Threesome y'all we WON!!! Time to read the novel(人 •ᴗ•)