Is one expiation for an oath sufficient to be released from a vow to donate a specific amount of money every time a sin is committed, if the vow was made in a state of anger and one is unsure whether it was uttered verbally or merely intended in the heart, and the donation has already been made three times?

For a vow to be binding, it is a condition that it be uttered by one who is capable of speech. What is doubtful regarding a vow is not binding, because the default is absolution from responsibility. As for a vow of obstinacy (nadr al-lajaj), its maker has the choice between fulfilling it or offering an expiation for an oath (kaffarat yameen). If the vow is conditional upon a phrase that implies repetition (such as: "every time"), then the vow is repeated with the repetition of the action. In each instance, the one who made the vow has the choice between fulfilling their vow or offering an expiation for an oath, unless they become unable to do so, in which case a single expiation suffices.