Without specific details on Chapter 3 of "Shounen ga Otona ni Natta Natsu," one can only speculate on its developments:
For those unfamiliar, the story follows a male protagonist during the sweltering break between his last year of middle school and high school. It is a coming-of-age narrative that leans heavily into the "Natsu-syndrome"—lazy cicadas, melting popsicles, and the sudden awareness of the opposite sex. Without specific details on Chapter 3 of "Shounen
By Chapter 3, the "innocent" summer games have ended. The protagonist is forced to confront adult emotions (and situations) that his younger self could not comprehend. Aku no Hana
Based on similar works (Kakukaku Shikajika, Aku no Hana, Oyasumi Punpun), Chapter 3 would: Oyasumi Punpun )