The problem is being in NAT 3- things are too closed down for online play to work properly.
You have a couple of options here- you could put the PS3 into the router's DMZ, which would put it outside of the protection given by the router. Your other choice is to go into the Gaming and Applications tab and into the port triggering. I here you would need to open certain ports used by the PS3:
* TCP Ports: 80, 443, 5223, and 10070 - 10080
* UDP Ports: 3478, 3479, 3658, 10070, and 50100
Once you enter all of these in and save the settings you may have to power cycle the router (turn it off for a minute or two and plug it in again). When you plug it back in you should be at NAT 2 and be ready to game.