Must the days not fasted from a vow to fast three days every month be made up if the month ends and a new month begins?

The vow to fast three days every month must be fulfilled, based on the Prophet's (peace be upon him) saying: "Whoever vows to obey Allah, let him obey Him." If you miss days of the vow due to illness or forgetfulness, you must make them up. It is safer to offer an expiation for an oath along with making up the missed days.