It is well-known that it is permissible to combine more than one intention of voluntary prayers in a single voluntary prayer, such as performing a two-Rak‘ah (unit) prayer with the multiple intention of it being the mosque-greeting prayer, the two rak’ahs for Wudhoo' (ablution) and the Sunnah of Fajr, etc. Is the same permissible concerning voluntary fasts? I mean, can I observe the six fasts in the month of Shawwaal, three of which coincide with the thirteenth, fourteenth and fifteenth days, such that I would hold a double intention for those three days the first of which would be to count them as three of the six fasts of Shawwaal and the second to count them as the three fasts of the thirteenth, fourteenth and fifteenth?

All perfect praise be to Allaah, the Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

You would get, Allaah willing, the reward for the six fasts of Shawwaal in combination with the reward of the three fasts of the thirteenth, fourteenth and fifteenth days of the lunar month if you observe only the six fasts of Shawwaal three of which coincide with the thirteenth, fourteenth and fifteenth days.

Allaah Knows best.