Yuni is a "high maintenance" girlfriend, who needs attention and love. When Fuuko comes up with just what she wants to hear, she wavers, because she's lonely and doesn't feel loved.
Nanase is the one who confessed, but then she lacks commitment to the relationship. She's much more invested in her club and in Volley than in her relationship with Yuni.
They don't have the same expectations in their relationship, which leads to frustration.
Then comes Fuuko. I'm not sure what she wants. It's like she's in "love" with Yuni because she's taken. It gives her power. Power that she probably lacked with her previous love (she said she never had the courage to confess) and she's much more possessive than Nanase.
If Fuuko continues to be this aggressive and Yuni doesn't put a stop, then Nanase will eventually have doubts and it will fester to high heavens. And then, Nanase has admirers in her club, so I wouldn't get past the author to add a 4th wheel that will muddy the waters.