THis issue is one of the reasons keeping all your data in one file instead of several is simpler to work with.
You have two basic options:
Set the other files to auto-enter an account and password when they open. This works if that particular file only needs the same access privileges for all users other than when you open the file to make design changes.
Write scripts so that you can create, delete or change an account by entering the needed data into global fields and then the scripts, set to run with full access privileges in most cases, make the needed changes in all three files. This requires using Perform Script to perform such scripts in the other two files--account and password info can be passed in a script parameter.
Take a look in the script editor at the script steps in the "accounts" group and you should find all the ones that you need to make this work.