Sankakkei no Kowashikata

Also known as: 三角形の壊し方, How to break a triangle, Sankakukei no Kowashikata

by Kabocha

Koto, Aya, and Erika were secretly in love with each other. Koto revealed her feelings for Aya, but a short while later, Aya disappeared as if she had been spirited away. Seven years later, Aya reappears to Koto and Erika, just as she was then... (Source: U-Next, translated)