If you look back at 2B, Brenda Hartford mentions an encrypted letter that was found in the suspect’s room at the boarding house he had been staying at. She mentioned that is was encrypted, and based of the case file, I am assuming that 3B is said letter. The “relatives in Brooklyn” hint in the case file probably serves to provide context to the letter, to help think of cribs to look for. I’m struggling a bit myself, but having a word document where you store all the decrypts and any notes/case files makes it easier to find context clues. I would also recommend looking online at how the cipher works (and possibly find the cipher type if you don’t already know it). I would also recommend searching the names of some of the characters, as the story is based off real people, to help find more context clues. Hope this helps
[We held this back a little bit, but it is an excellent post. Thank you, Harry]