There is no real capability in WSS to do the kind of Shared Document library
that you are envisioning. Library are always in the context of a specific
site. The best way to accomplish what you want to do would be to write a
custom web part that would aggregate library contents, with security
trimming, from each site  in the site collection and display it in the web
part on a given site.

I am new to SharePoint/WSS.

Can you give me more details on how to do this or where to look to find out how
to do it?

Would we be able to upload docs  to one place on the main  site?

Each client  has their own sub-site of the main site.
Would each sub-site have a webpart that would show  only the docs that they can
see?

Sharepoint security trimming does just that automatically. If a user  does
not have rights to read something they will not  see it.
This-
blogs.msdn.com/.../...epoint-end-user-security.asp\
x
Gives a full breakdown of how it works.

Part 1. Introduction says that the information
Applies to: Microsoft Office SharePoint Server 2007, Windows SharePoint Services
3.0

However, Figure 1 shows a page that says Welcome to Microsoft Office SharePoint
Server 2007 and all the instructions are geared toward Microsoft Office
SharePoint Server 2007.

The first two steps of the instructions are:
1. Open your browser and navigate to the Central Administration Web site.
2. click  the Application Management tab, and then click Create or extend Web
application.
--- end excerpt from instructions ---

We are running  WSS 3.0, which does not  have a Central Admin Web site, or an
Application Management tab.

The instructions also mention Granting Permissions to Create My Sites.

WSS 3.0 does not have My Sites.

There are no alternate instructions for wss  3.0.

Are you sure this can be done in WSS 3.0?

If so, there should be separate instructions for WSS 3.0.

Was 3.0 doesn't have MySites, but it does have a Central Admin website with an
Application Management Tab.