What is the ruling on taking medication prescribed by a doctor three times a day for three weeks, knowing that this conflicts with fasting Ramadan?

If you fear harm from not taking medication, and you cannot take it except during fasting hours, then it is permissible for you to break your fast, based on Allah's saying: ﴿So whoever among you is ill or on a journey [during them] - then [he is to fast] an equal number of other days.﴾ You should then make up the days you missed fasting. It is advisable to consult a doctor about the possibility of taking the medication outside of fasting hours.