I'm not aware of a file sharing only tutorial for 3.0, so let's see if we can navigate the proper setup here.
If we begin with the basics, you must have at least one network card on the server, and that has to have an IP address--that is, it has to be a configured network card. Its address can be a static address (assigned within Zentyal or by a DHCP server on your network) or it can be a dynamically allocated address (issued by a DHCP server on your network), but it must have one. That's configured in Network > Interfaces (I think--working from memory).
Because of Zentyal 3.0's use of Samba 4, I would at least leave the default settings on the general settings tab of the file sharing module, even if the server will be used only for file sharing. I'm not reading the manual page the same as you, but in any case, I think Zentyal will need some values in those fields, and the defaults are a good place to start.
This is a major change from Zentyal 2.2 so it will take some getting used to. There really is no file sharing only configuration for Zentyal 3.0 as far as I know. Zentyal will want to be a domain controller as well, though you should be able to simply ignore that service on your network.