What should I do if I swore to fast for a hundred days if I did a certain thing, then I did that unintentionally? Do I fast for three days or a hundred?

Your oath: "If I do something, I will fast for a hundred days" is an oath of obstinacy (nadhir lajaj), and you have the choice between fulfilling it or atoning for it. If you fast for a hundred days, it will suffice. If you do not fast, then you must perform an expiation for an oath (kaffarat yameen). The expiation for an oath is to feed ten needy persons, or to clothe them, or to free a slave. If you cannot find any of these, then you must fast for three days. Fasting is not sufficient unless you are unable to feed or clothe.