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NAT type 3 means that one or more of the ports that PSN uses are being blocked in the network. They may be blocked by the broadband modem or router, or in the case of mobile broadband, they may be blocked somewhere else. If the ports are blocked in the modem or router, then you should be able to access the router management interface using the PS3 web browser (follow the directions for the router), and disable the firewall in the router (which you don't need if all you have is a PS3). If the port is being blocked somewhere else in the network, then all you can do is complain to your ISP, and hope that it changes the network configuration for you.