@SUBARU,
@Abedeus - I did say it was difficult to translate. Anyway, when Miku first transformed into a human, the first thing she saw was Suzu's sad face. She "imprinted" on that, which according to her was like falling in love at first sight. So she's always had a thing for Suzu. But Suzu, for her part, has never recognized it because she has been head over heels for Yuu since long before she ever met Miku. Now that Yuu has returned, Suzu is happy. Miku sees Suzu happy, and is happy for her, and yet at the same time recognizes that her own love for Suzu will never be returned in kind. Miku recognizes that part of this was her own "imprinting", but feels helpless about her own feelings. So what is the best compromise? Support Suzu's love for Yuu, and since Yuu will eventually be considered a community resource shared by all, she expects she will be able to satisfy her physical needs eventually through him. Hence the "threesome".
Her desire to see Suzu happy leads her to ask Yuu to give preference to Suzu's needs.
Is this complicated enough? I suspect things will get more complicated yet before the end.