Practicing a writing format can be really useful and helpful even if the topic/subject is unconventional for that format...
In the letter, I would expect Nora to have as many questions as she might have items of news to share. How are her children doing? What are their new interests and hobbies? Etc.
Since you have to make it a business letter, maybe you could write it as if Nora is applying for a job at the bank Torvald managers. You could start with a brief introductory biography, including her schooling and any experience. That would also give you the opportunity for her to talk about what she's been doing in the past three years that would make her qualified for employment. You could also include some of her philosophy on women's place in society and how women are able to think as deeply as men and worthy of partnership with men.