Is the mac wireless instead of wired, as compared to the other two pcs?
No, it is wired, as both PCs, connected to the same network hub…
any vpn software or third-party firewalls on the mac? Anything else strange about the mac?
What you describe doesn’t seem possible. If you aren’t seeing it from itunes or bonjour browser, it can only be a few things —
the mdns responder isnt’ running on your mac.
you are firewalled, or some other networking issue is blocking multicast
That’s really it… I can’t think of anything it might be other than a fundamental networking thing.