Check a couple of things:
Make sure they are all part of the same workgroup...
Right click my computer and select properties.
Go to the Computer name tab and verify that all the computers on your network are in the same workgroup. If not change and you may have to reboot.
The computers on your network need the same IP addresses except for the last 3 numbers i.e. 192.168.1.xxx The first 3 octets need to be the same the last need to be different. You can check this by clicking Start, then click Run, type in cmd, in the dos window type ipconfig /all
Verify on each pc that the IP addresses are similar. If you have a router then you most likely should be using dhcp.
You may also need to turn on file sharing here is a link to do that.
File sharing link