Is it permissible for a Muslim to combine voluntary fasts, such as fasting every other day, with observing the fasts of Mondays and Thursdays and three days of each month, or must one confine oneself to a specific fast even if it is at the expense of the recommended days?

Fasting one day and breaking fast one day is the best form of fasting, and the most beloved to Allah Almighty. There is nothing better than it, even if a person is able to fast more. However, if the day he is supposed to break his fast coincides with a day whose fasting is sunnah (recommended), such as the Day of Arafah or Ashura, then it is better to fast it. This does not diminish the virtue of fasting one day and breaking fast one day.