What should I do with my wife, whom I made an oath upon not to go to a certain place, but she violated my oath and went, and now I am obliged to fast three days as expiation for the oath?

A Muslim wife must obey her husband in what is good, and her disobedience is considered an act of defiance (nushooz). You should advise your wife and explain that her obedience is compliance with Allah's law. The expiation for breaking an oath is feeding ten needy persons, or clothing them, or freeing a slave. If you cannot find these, then it is fasting for three days. Fasting is only permissible if one is unable to feed or clothe.