What should one do who vowed to fast ten days and fasted three, then committed a sin? And what is incumbent upon him regarding his fast? And is expiation obligatory upon him? And will Allah accept his vow?

Masturbation is forbidden, and invalidating a votive fast by it is forbidden, due to the obligation to complete it once it has begun. It is sufficient for you to make up the day you invalidated by masturbation, because masturbation, if it results in the emission of semen, invalidates the fast. You are not required to repeat the days of the votive fast that were not invalidated, nor are you required to offer expiation, even if intercourse occurred during a fast other than Ramadan. Increase your seeking of forgiveness and repentance.