Helllo Furture Zone,
You mentioned "This small office have 3 sep client license purchased", does that mean you ONLY have 3 computers running the Symantec Endpoint Protection v12.1 antivirus?
Also, you mentioned that the license is renewed and that you received ONLY 1 ".slf" file.
Usually when a SINGLE ".slf" goes on the SEPM (Symantec Endpoint Protection Manager) which allows the SEPM to now recognize the number of clients to manage. Usually customers will create an "Unmanaged" client installation packaage and deploy it on the remote computers. This install package will already be licensed (as it was created from a licensed SEPM)
Yes, you should attempt to copy the same license file in the Inbox for all the three clients. Hopefully it should work (considering your scenario with 3 client licenses purchased and now renewed).
If not, i have another workaround (this might involve installing the SEPM) in your network if you do not already have one. I will give you the details, once i hear back from you.
Good Luck!