Isourousaki no Sanshimai ga Ecchi na Training wo Motomete Kuru

"I am a former decathlete, and in exchange for helping me get back into the sport, I am asked by the school's director to help his three daughters compete in the national championships."

I, Itsuki Juuouji, a former decathlete, receive a request from the school's chairman in exchange for support in returning to competition, but all three of his daughters are troubled children filled with worldly desires!? Sadistic, masochistic, and exhibitionist... These three sisters direct their naughty desires towards me that they can't release in their school life. Is my life in danger if we live together!? I'll let you release your naughty desires through training that will make the most of your unusual sexual tendencies!

Later on, I regret that I underestimated the three sisters' sexual proclivities!

(Source: Overlap Bunko, translated)