I do not believe that my Home Gateway will not forward from the external IP to the 192.168.2.57.
Ummm... Let me think past the double-negative...
You will need to set-up the 3Com to forward any port 80 traffic coming to it to the webserver. Then, you can just use the external IP of your 3Com. You don't really need a domain-name... It's just easier to remember and type.
If you can't set-up the 3Com to forward port 80, then you are out of luck... The external IP is the only thing reachable from the internet. The 3Com would work as a router going both out from the network to the internet, and in from the internet to the webserver.
You will also need to be sure that your ISP doesn't block port 80. If they do, then the requests will never reach your computers.