So, a bit late to the party, I just discovered this manga today so had to catch up.
First of all, Nanase is (almost) blameless in this situation. Her only fault is being insecure and inexperienced. She realises her faults over the course of manga, and tries (but not always succeeds) to improve herself. Yuni on the other hand is insecure and clingy. She wants Nanase to basically read her mind and doesn't make a serious effort to really connect, she also only wants praise and demands that her lover gives her all her attention. Nanase's "fault", at this point, is not being totally absorbed by her girlfriend.
Nanase slowly opens up, tries to speak with people she trusts about her relationship. Yuni doesn't want to go public, despite being annoyed at Nanase for exactly the same thing. She demands that Nanase sacrifices everything for her, but is unwilling to sacrifice anything in return. She wants to have the cake and eat it too.
As for Fuuko, well. She's shown constantly reading a book about tragic romance inspired by a cordyceps fungus. A parasitical fungus. Something that devours from the inside to grow. She's devouring the other two's relationship, and gets off on it too. Thing is, there's nothing to prove that Yuni's special to her. She just found the alt account and stalked the owner. If she gets off on the parasitical part of the relationship, there's nothing to stop her from jumping ship once Yuni and Nanase break up. After all, manipulators are really good at making you feel that you need them, and it's pretty obvious that's what's happening here.
But anyway, I'm reading too much into it and it's still somewhat hot. New chapter should be released soon, too.