so i reread a fe12 lp and realized that nyna mysteriously vanishes in her ending, and camus canonically returns to valentia. i think your idea of a poly between the three is canon.
No matter what “Sirius” said she knew he was Camus. It’s obvious she went on a journey to seek him out in her ending. I like to imagine that through sheer persistence she did manage to track him down to the village in Valentia where he lived as “Zeke” with Tatiana.
He’d try to deny he was Camus but I feel like Tatiana would already have an inkling of why he’d disappeared for several months and force him to admit it. Nyna and Camus would catch up a bit, awkwardly at first but soon hitting it off again--Nyna, now that she’s confirmed that Camus is married, isn’t cruel enough to try to weasel between their happiness, but she’s still glad she’s gotten to see him again. Camus and Tatiana would offer to let her stay with them for a little bit, at least until she figured out where to go from there.
Sure, Tatiana isn’t exactly thrilled that a woman who is in love with her husband and who she probably feels could potentially take him away from her has suddenly showed up out of the blue, but she always knew and feared the possibility. And moreover, she’d feel sympathy for Nyna. She can see how much life has worn her down, and her sincerity and kind nature--it’d probably activate her nurturing and protective instinct in a similar way to how Camus washing up on shore did. Besides, she can hardly blame the woman for falling for Camus--she still remembers how quickly and strongly she did, herself.
Time would pass, and it wouldn’t be long before there would be love bubbling right underneath the surface between Camus and Nyna again, but both keep a strict platonic wall around their interactions out of respect for Tatiana. Tatiana definetly notices this, and sees how happy Nyna makes her husband. She doesn’t exactly feel jealous, because Camus still loves her as well and goes out of his way to show it to her. Just, perhaps, a bit nervous. Both are expecting that soon Tatiana will declare that it’s time for Nyna to move on, but she never does.
Nyna and Tatiana are growing closer throughout this time, as well. They certainly have an immediate point of connection in discussing Camus’s past and present, learning different sides of him from one another. But soon their conversations expand to other areas as well, each other and Archanea and Valentia and whatever is the matter of the day, and they become surprisingly good friends. Nyna is looking notably healthier and happier and brighter than when she first showed up in the village, and Tatiana is delighted for it. She has so much love in her heart ;_; The two probably run errands together and occasionally travel to festivals with Camus to have a good time.
At some point, Tatiana would probably seriously start reconsidering their arrangements. She can tell Camus and Nyna love each other but are refraining from expressing that because of her, and she doesn’t like to see them restrict themselves as such. Pushing through a lot of uncomfortable feelings and insecurities, her care for Camus and especially her new care for Nyna win out, and she offers Nyna the idea of, ahem, “sharing” Camus. Nyna would probably be totally caught off guard by this, and actually try to argue back against it--no, Tatiana should value herself more, no, she was the one who married him and Nyna had no right to intrude but Tatiana would insist. Nyna is so moved by this kindness and understanding. Allowing Camus and Nyna to love each other is ironically itself a large act of love coming from Tatiana.
Tatiana watches as Camus and Nyna finally share their first kiss, and she immediately knows she made the right choice. It honestly couldn’t be more than a day or two when Nyna is just thanking Tatiana for all her care and kindness and for how happy she’s made her until the two find their limbs tangled up in each others and their lips pressed together. I’d imagine in a few months they hold a new wedding ceremony that officially makes Nyna a part of the two’s union. Camus loves his wives, Nyna adores her first love and her new love who shined so much new light into her life, and Tatiana thanks the gods every night for her two beautiful Archanean loves who it almost feels were delivered to her through some sort of divine providence.