That's hardly NTR. If either other chicks were getting dicked by another dude then I'd be piss about it. Couldn't care less if MC's girls are banging each other, that shit is hot and could open up threesomes later on.
My brother in Christ, I’m not some goon scholar. I barely even know what the hell NTR is outside of one or two hentai where the main guy’s girl gets fucked by someone else. I just thought it might count as slight NTR because the same dynamic is happening: Lin likes Rin, but Rin’s getting railed by the MC while Lin’s left dreaming about her. Which, I’m sure, has a lot of similarities with the thought process of a cuck. I don’t even get why you care this much about what counts as NTR when you’re literally gooning over fictional characters smashing each other.