Osananajimi to Amaaku Ecchi ni Sugosu Houhou

Sousuke has a childhood friend, Asuka. His family often moves due to his father's business. One day, Emily and Kotone, his other two childhood friends, start to live with his family. Emily finds a diary, which Sousuke used to keep when he was a child, and some erotic games they played are written in it. Inspired by it, Asuka, Emily and Kotone start to fight to decide who is the real Sousuke's childhood friend by playing the erotic games they used to play...