Hi George,
If at least one of those Macs is running OSX then there might be a couple of ways you can go.
OSX has Samba and I believe you can set the Mac to be the samba server and set up the other machines with Samba clients. If the PCs are running WinXP, then there are samba clients already installed. I mention Samba first because now we can get it out of the way..

It really is cool when it works but it can be a bear if you've never done it before.
So, the other option is:..(again if one of the Macs is OSX), you can turn on the FTP server that comes installed on Mac-OSX. Designate a public folder on the mac and build folders inside it. I would create a folder with the name of each machine on your network. Each machines' files will go in its respective folder. Then use an FTP client from each of the other machines to "talk/transfer" files between them and the Mac. In this scenario, all files will have to go to the "central" mac, the one running the FTP server, but each of the other machines can talk to it.
possible example: G4 = FTP server
move file from PC to G3 by placing that file on the G4 from the PC, and retrieving it from the G4 with the G3. Going the other way 'round should work too.
Your PCs should be able to talk to each other through their native protocols so you shouldn't have to move PC files this way unless you're running a mix of OSs..win98/winxp/win2000. If the G3 and G4 have different Mac systems, then FTP may also be the easiest to transfer files between them.
We have several machines here. I use Linux on a desktop, Hubby uses WinXP, I have a winXP laptop, two kids use a win98 machine and one kid uses a G4-OSX. I FTP files to the mac from _any_ machine. The Mac can FTP files to the central Linux server that is also running FTP. Both winXP machines talk to each other natively, but they also FTP to the central linux machine, (or the mac), and the win98 machine can FTP to the central linux server, the mac, and it talks to itself..*grin.
If I need to get a file from my Linux desktop to Hubby, I send it to the central linux server and hollar across the 6 foot span of distance between us for him to go get it from the central server.
*grin..not the cleanest way of doing things, but we trade files only rarely and FTP is already available, and it talks to ALL systems without trouble.
IF you decide to try the FTP route, make very, very sure that it isn't accessible to the wild,wild,web. If you have a router holding your internet connection open, then make sure all FTP access is denied from the outside world. Make sure port 21 is closed.
The FTP method may sound confusing, but it really is pretty straight forward. Ask if you have questions.