Is your LAN connection via a router? If it is, check that DHCP is enabled. If it is then there might be an option to see all connected devices. You can get their IP address from this screen.
If this option is not present then click Start >> Run and type in cmd and hit Enter.
In the command prompt type in IPConfig andh it enter. Look for IP address or IPv4 address.
Do this on each pc and note their IP addresses.
If you're NOT using a router and just a switch you're going to have to manually setup static IP addresses in the same subnet so they're on the same network. I won't go into detail unless you need help on that.
Your problem seems like Comp3 has is on a different subnet or permissions aren't properly assigned to the shared folder. I suspect its the former though.