What good fortune has come to the Helmer household?
In Act I, the good fortune that has come to the Helmer household is Torvald's promotion. Torvald is pleased because he is advancing both financially and socially. Nora is pleased because this means that she can get out from under the illegal loan she procured in order to save Torvald's health. She will finally be able to relax the tight budget she has had since the beginning of her marriage. In Act III, the good fortune that has come to the Helmer household is regarding the blackmail that Krogstad has been attempting on Nora. As Krogstad is now happy in love, he has given up his intent to blackmail Nora and Torvald into returning his job.
That depends entirely upon how you view the play, and which section of their lives you're asking about. In a sense, Torvald is lucky that his wife loved him enough first to break the law to borrow money, and then to work hard to pay back the loan. They are fortunate that he recovers from his illness. Torvald would say it is good fortune that Krogstad decides against exposing Nora's secret.