Mizuki and Kazuya, childhood friends, are 3 years apart. It's not really a so-called "age gap" but it makes some distance between them during their youth. From their childhood to the present, the story will be told about these two who are 3 years apart. (Source: Square Enix, translated)

Sannensa is a web manga released on the author's Twitter account from May 5, 2017 to April 21, 2020, and published in print by Square Enix from August 22, 2018 to April 22, 2020.