It'll be extremely complicated, but Envoy can do what you desire. The hardest part, believe it or not, will probably be integrating the real Amiga network with the emulation on the PC. As I recall, WinUAE doesn't emulate a specific piece of network hardware, it just presents a network interface that allows TCP/IP to work transparently. That means no SANA-II driver for Envoy to use. Don't know if this is still the case, but there are some other tricks you can try.
You'll definitely need to make some Parallel cables. You can link the 500s via Serial and then use the Parallel port on one to link to the 1200. (The floppy-port networking interface is a full piece of hardware, not just a cable, so finding one now will be an extreme challenge).
With some clever gateway configuration, you should be able to get Envoy to access all machines. The 1.3 one will require some extra thinking as Envoy is Kickstart 2.x+ only. A 1.3 and 2.x compatible network program (check Aminet) could be run on both 500s (mounting the 1.3 drives on the 2.x one) and then Envoy could re-export the networked drives to the 1200. Performance will be abysmal, but it could work.
The 1200 could in turn re-share everything via Samba (using a PCMCIA card to access standard Ethernet), which SMBFS on UAE might be able to handle. If not, use Windows to mount the drives and then add them to UAE.
