Yesterday Once More Dousou Kai

A young group of friends meet up for the first time in a very long time after finishing high-school. One of them remembers how once ago a girl left him Daisies and a love letter inside his locker, back in the high-school days. He has three suspects of who this secret lover could be and so there is only one way to know... by having sex with all of them. (Source: AniDB)