Is it obligatory to fast three days for each sin that was repeated after the vow, or is a single expiation for an oath sufficient, or is it only obligatory to fast three days for all of them, knowing that the sins were repeated many times?

A vow to fast three days upon committing a sin, with the intention of restraining oneself from it, is a vow of lajaj (obstinacy) and anger. Its ruling is that of an oath. The one who made the vow has the choice, according to the majority of scholars, between fulfilling it or offering an expiation for an oath (kaffarat yamin). The evidence for this is the Prophet, peace and blessings be upon him, who said: "There is no vow in anger, and its expiation is the expiation of an oath," and his saying: "Whoever swears by walking [to the Kaaba], or by offering a sacrifice, or by giving his wealth in the way of Allah, or to the poor, or to the curtains of the Kaaba, his expiation is the expiation of an oath." As for vowing to Allah, it falls under the ruling of a vow of lajaj, and this vow is repeated with the repetition of the sin.