Whose Letter Is This?

Wakui is living a lackluster college life. His only pleasure was correspondence.
The correspondence that started with a letter he found in the university library became the center of Wakui's life.
However, one day, he received a letter containing information that his pen pal was not supposed to know.
Wakui suspects that the correspondent is someone he knows, and he begins to suspect the three girls he is involved with.
However, the three girls are all good singers…