Shared File Access

I have three PCs on the company network -- no firewalls
in between them.
Two are recognized by servers -- hostname & password.
The third is not and will not be, it has its own users
and passwords -- on the network, but stand-alone.
The PCs recognized by servers (hostname and password) can
connect to each other by setting up user permissions --
using server recognized username and passwords -- always
reconnects.

The third PC is a problem. Sometimes I can make it work
by disconnecting one of the shared folders, rebooting and
mapping agin. Now it simply will not connect. I use the
remote PC valid PCname\folder and User & password (as
defined on this third PC), or PCname\Folder Administrator
& password (also as defined on this third PC). I get the
message about "already connected as another user" etc.
What does this mean, I am disconnected! It has been
diconnected and re-booted. In the past disconnecting one
shared folder and rebooting the PC attempting to make the
connection to the third PC would allow me to (re-)map the
shared folder, then the other shared folders would ask
for user name and password when I tried to open them --
and they would open. Now I cannot get it to work at all.

I can see and always reconnect to a printer on this third
PC withoput a problem, it is just the folders. I can
also do a remote desktop to this third PC, but of course
that does not allow me to transfer files from the third
PC back to the one I am on (unless there is something
else I do not know).

Any help appreciated.