Is it permissible to fast the Ayyam al-Bid (the White Days) and three days at the end of the Hijri month?

Fasting three days every month is established in the Sunnah. It is permissible to fast them at any time of the month, and it is recommended that they be on the White Days (the 13th, 14th, and 15th). There is no evidence for the recommendation of fasting the last three days of the month, so one should not restrict oneself to them.