The ordinary way to setup a proxy under GNOME 3 would be to go to Network->Network Proxy and chose the type of proxy option you want – Automatic/Manual.
If you do specify Manual proxy you will be surprised to know that there is no place to provide authentication details such as username and password. The Previous version did support such feature but for some unknown reason its not present in GNOME 3.
A useful work around is to use gsettings in the terminal. Steps as follows:
“gsettings list-recursively org.gnome.system.proxy” – It states the current proxy configuration details.
“gsettings set org.gnome.system.proxy.http authentication-user USERNAME” – Assign the username for proxy.(replace word in caps with your choice)
“gsettings set org.gnome.system.proxy.http authentication-password PASSWORD” – Assign the password for the proxy.(replace word in caps with your choice)
“gsettings set org.gnome.system.proxy.http use-authentication true” – Finally activate the authentication.
And Viola! Its Done 🙂