What is the ruling on one who masturbated in Ramadan three minutes after the Fajr adhan, thinking that the Fajr adhan had not yet occurred, and then it became clear to him that the adhan had indeed taken place three minutes before his act?

Masturbation requires repentance. If one masturbates after the dawn (Fajr) enters, their fast is invalid and they must make it up. However, if one masturbates before dawn, their fast is valid, based on the Hadith of Aisha (may Allah be pleased with her): "The Messenger of Allah (peace and blessings of Allah be upon him) would wake up in the morning in a state of major ritual impurity (janabah) due to sexual intercourse, not from a wet dream, in Ramadan, and then he would fast." If someone doubts the entry of the time and masturbates, their fast is valid as long as it is not clear to them that dawn has entered.