Koisuru Onna-tachi: Triple Ecstasy: Keiren (1988)

Yoko, Shoko, and Kyoko are three female college students. Yoko has a boyfriend named Hiroshi, but when she hears that Shoko is having an affair, she becomes anxious. She also dreams of having an affair. Yoko approaches her professor Maehara to have an affair. Kyoko ends up sleeping with Yoko's boyfriend Hiroshi by chance. And then the three of them.