Is it permissible to switch from the expiation of a vow to fasting due to not having sufficient money and fearing one's father's inquiry about the reason for spending the money, and how are multiple expiations calculated over time, and is it permissible to fast three days weekly for uncounted expiations, and what is recommended in this situation?

The one who makes a vow of "lajaj" (a vow made in anger or to prevent oneself from doing something) has the choice between fulfilling his vow or offering an expiation for an oath. You are obligated to perform only one expiation, even if the sin is repeated, unless the wording of the vow indicates repetition. Your fasting instead of feeding depends on your financial ability; if what your father gives you is a monthly allowance for your needs, then you are not required to pay expiation from it. It is permissible to switch to fasting instead of feeding for one who does not possess more than what suffices for his and his family's sustenance for a day and a night, or one who does not possess what suffices him for a year, like someone eligible for Zakat.