What is a Muslim supposed to do if he is fasting for the expiation of his oath to Allah and during the last day of the three consecutive fast breaks his fast in the middle for no reason?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

It is not a condition to fast consecutive days for the expiation of the oath. Allah Says (what means): {then he should fast for three days.}. So Allah did not say “consecutive” days. Therefore, it is permissible to break the fast in order to resume some other days. So, now, you should fast the third day only.

Allah knows best.