Is it permissible to combine the supererogatory fasting restricted to two or three days each month with the intention of facilitating marriage, or must other days be chosen for this purpose?

It is permissible to fast on Mondays, Thursdays, and other recommended days with the intention of obeying Allah, seeking reward, following the Sunnah (definition), and also with the intention of curbing desire through fasting. This falls under the category of "sharing intentions in worship" (al-tashreek fi al-ibadah), where one of the acts of worship is not intended for its own sake (such as fasting to curb desire), while the other is intended for its own sake (such as fasting on virtuous days). Similar examples include fasting for Allah and for diet, or performing ablution or ghusl (definition) to remove impurity while also intending to cool down.