For all computers on a workgroup:
. Add the usernames and passwords of computer A, to computers B, C
. Add the usernames and passwords of computer B, to computers A, C
. Add the usernames and passwords of computer C to computers A, B
Workgroups are annoying this way. XP as a client does a better job of caching stored credentials, but essentialy in a Workgroup setting you need to make all of the username/password entries match.
The security in this instance them comes from the basic Windows "Share" facility to restrict access to any given share by username (since you have now enumerated them as local username/passwords, this becomes easier) and through the use of NTFS permissions by user and access right (similarly, since they are now all defined as local users).
There is no facility to enumerate workgroup members when creating share or NTFS permissions. You need to by hand create the synchronization of all username/password combinations to fully use Windows security in a Workgroup -- particularly in a mixed OS setting as you describe.