The IP addresses you have given them will not both connect to the same address. You must have all the numbers the same until the last set in the IP address. When one computer is at 192.168.1.xxx and the other is at 192.168.2.xxx, they are on totally different networks.

You will probably need to reconfigure the Powerbook to the third group being 192.168.1.x - with x being something other than 3, since then it would be identical to the iMac.

Rick