What is the ruling on fasting the last of the three days for expiation of an oath, given that the questioner did not pray Dhuhr and Asr on that day until after the onset of menstruation?

Your fast is valid and you are not required to make up for it. Your obligation regarding the expiation for the oath (kaffarat al-yameen) has been fulfilled if you fasted while unable to feed or clothe [the poor]. However, you committed a grave sin by missing the prayer and letting its time expire. Therefore, you must repent sincerely and never return to this sin. You must also make up the two prayers after you become pure.