Is it proper to staple a 3 page business letter?

Yes, it is generally acceptable to staple a 3-page business letter, especially if the pages are intended to be read together. However, ensure that the staple is applied neatly and does not obstruct any text. Additionally, consider using a paper clip instead if you anticipate that the recipient may want to separate the pages for any reason. Always follow any specific guidelines or preferences of the recipient if known.