If you create an account on device 3 for user a and share the documents folder using the advanced sharing option, including file permissions, that will give them access, it will ask for credentials for the first logon but will not after this. The additional user on the device 3 is a requirement of workgroup networking
The major problem with your set-up is that you have all your users as administrators in a peer to peer system (workgroup) the local administrator has the ability to change any settings made by any other administrator. this would mean that as an administrator, person f would also be able to reset permissions on the folder to gain access.
The only way to fully secure this system would be to have the other users as standard users who would be unable to access the permissions
You would have one system administrator for the whole system, with an account on all devices, who could then assign permissions for access of the other users to all files.