I'm late to the party but I finally faced those... tags.
Eri seems to me like a calculated person. In chapter 1 when she said yes to Kyou, it's not because she likes him back but because she thinks Fuyuki likes him. Author made it a point to show Fuyuki's warm relationship with Kyou--him relaxing at her house, her watching that scary movie because he lent it to her, him easily agreeing to Fuyuki in that map discussion--all in chapter 1 and involves Eri being there. Eri wasn't even happy when she said yes to Kyou at the end.
In chapter 2 and 3, it's quite apparent that Eri has some feelings for Fuyuki. She wasn't lying when she said that she's having the most fun watching a boring film when she's with Fuyu or that the latter is her most important person even when she's dating someone. She's even attentive to Fuyu's likes or what she has to say.
In chapter 3, we see a bit more of their thought processes when it comes to each other.
For Eri's, it started when they were younger with her sharing all about her dating activities to goad Fuyuki for some sort of reaction--to give a hint of where she's at in Fuyu's heart--but the latter would never falter in her support or reassurance of her instead. She didn't immediately share about Kyou because she feared that Fuyu would be angry enough to cut off their relationship, which she feared the most. As for why she said yes to Kyou instead, she could've feared what would happen if she rejected him, as he could very well aim for his other friend instead which he seems closer with.
Meanwhile, Fuyuki's thought process towards Eri was to always support and reassure her; and to never show any hint of jealousy, not even when Eri was asking her if she'll ever get lonely if she starts dating.
TL;DR Eri is a calculated person who would do anything for Fuyuki not to be with someone else while Fuyuki is a selfless person who would do anything for Eri not to hate her. They both think they're in the friendzone, that's their mistake.
Honestly though, I would very much like for Fuyuki to date her senpai and to be extra open about it towards Eri, just to give the latter a taste of her own medicine. Since Eri never knew about senpai's existence, the latter's not accounted for in her equation so all the more reason to look forward to senpai's future performance.