Must missed fasts due to menstruation, pregnancy, and intercourse in Ramadan be made up, or is expiation obligatory, or what is incumbent upon one who missed fasting these days and regrets it?

You must hasten to repent and increase in righteous deeds after having fallen short in fulfilling your duty to Allah. For Allah accepts the repentance of whoever is sincere to Him and replaces their evil deeds with good ones. You must make up the days you missed from Ramadan due to menstruation or pregnancy, and along with making them up, there is a minor expiation, which is feeding one poor person for each day, due to delaying the make-up. As for breaking the fast during adolescence before puberty, there is no sin or make-up required for it.