Windows will default to assuming you’re on a Public network. This will configure the firewall rules as if you were in a public place like a coffee shop on public WiFi. You don’t want this on a properly configure home network. Make sure that all the PCs think they’re on a Private network (remembering to reboot any PC you need to change network settings.)
Assuming they all think that they’re on the same network, then you should confirm they think they’re all in the same Workgroup. Windows will apply a default Workgroup, but I prefer to make sure I set it to something I control. In my case, I use “Home”, short and sweet and easy to type. While you’re in the dialog that has these settings, also check the length of PC name. The length of the PC names will also impact their discoverability, so make them on the short side (I think less than 16 chars.) (Again, you’ll need to reboot with any changes.)
On each PC, on the CMD line, get it’s current IP address with the ipconfig command. From the same command line, ping the other PCs via their IP address. If this fails, you have significant networking or firewall issues that need to be investigated before any other troubleshooting.
Assuming the pings succeed, then move on to pinging by machine name. This will see if the network can resolve the machine name to its IP address. If this fails, then you have an avenue of investigation.
If the pings by machine name succeed, then I suspect you have some issue related to configuration on the PCs. For example file sharing is not properly enabled.