In which world? Sure, the game and the story is pure fiction, but the devs tried to make the characters realistic. No woman in their right mind accepts a throuple as a serious relationship. That's just porn fantasy.
yet there was a concern to fit it in as naturally as possible....
I honestly don't quite understand what you're talking about, never mind Jaye + Mallory as that would be fan service (what in an AVN isn't?), but how was the Amanda + Mallory trouple constructed? with no premise, they are very close friends so they clearly have no problem having a relationship with the same man, who only one of the 2 knows well, but the other is her best friend so it could only end up that way, so natural and realistic.
With no real premise about the relationship (or does Amanda secretly touching MC's dick count as a premise?) and no particular consequences.