What IP addresses do the two "client" PCs get from the "host" (which presumably has 192.168.0.1)?
Has any firewall software been updated to allow access to the IP address of the new PC?
I have seen it suggested in these forums (though I remain to be convinced) that Internet Connection Sharing via an Ad Hoc wireless network only supports one "client" PC. In any case, this wouldn't explain your inability to see shared folders and you say that visiting laptops have access OK.
You could try allocating a fixed IP address to the new PC and enter the Host's address for the Default Gateway and the DNS server.