What is incumbent upon someone who vowed to fast three days if they committed a specific sin, and then committed this sin two to three times? Is it sufficient for them to fast the three days they vowed?

A "nazar al-lajaj" (a vow made in anger or to prevent oneself from doing something) necessitates either an expiation for breaking an oath (kaffarat yamin) or fulfilling the vowed act. Therefore, you have the choice between fasting the three days you vowed to fast or offering the expiation for breaking an oath. The expiation does not repeat with the repetition of the sin, unless the wording of the vow explicitly requires repetition.