Recover 3 Encoded Cards

Journal

Lord British's castle has been invaded, and Captain Marisha of the guards asked for my help. She's heard the invaders have copies of the 3 encoded cards for a safe within the vault. The captain is worried they'll get inside. She asked me to bring them to her if I find all 3 cards. Also, she made it very clear that she wants me to stay out of the vault and not use the cards to open the safe. I don't think I could even get in there, unless I had Lord British's vault keys...