“OT3” (“one true 3some”), a play on “OTP” (“one true pairing”), means “a polyamorous ship involving three characters that is a personal favorite of a fan” (
Wiktionary's definition, that I like).
Let me reply with another acronym: Tksm! Thank you so much! Though I don't remember reading this manga 😵💫
Edit: now I remember! I was reading everything Niosioisin had here as a fixation with his writing style. I think he's all style and no "substance", and I mean it as a good thing.
That's because by "substance" I mean "morals". He doesn't write in a moralizing way, ever. And only in terms of his character's quirks and motivations, which make the whole thing hard to remember to my moralist brain.
Nonetheless I live it very much, sorry for gushing out of nowhere.