This is not possible from the phone itself. You can only directly connect to the webserver on the phone if it is on the same network as the PC you connect from.

For 2 different networks you would have to configure the router between the networks to route the traffic it gets on a specific port to port 80 on your phone. This is what you usually call "Port Forwarding" in the average home router, which can be used to make services in your network available to the internet.

For 3G Network the equivalent to your home router is under control of your Mobile Operator, so there is no way to do this kind of port forwarding.

Another possibility would be to establish some kind of VPN to a server on a public network (== Internet) which is set up to forward traffic on one of it's ports to the phone via the VPN-Tunnel. But this kind of setup would require some server under your control on the internet and the skills to configure such a setup