My bad for replying a bit quick.
Can you try to ping the main PC from the other two? If ping works, then
1) might be MS Network problem. Here is MS link:http://support.microsoft.com/kb/134304
This second link seems to be better:http://networking.nitecruzr.net/2005/05/troubleshooting-network-neighborhood.html
2) something is blocking the other 2 from seeing the main. A firewall or some sort of software you recently installed? Did you do anything unusual around the time it stops working?