Multiple PS3's in a network may require some network configuration.
NAT Type 2.[]
In order to play multiplayer games online your network must be configured as a NAT type 2. You can check this using the Network Settings section of your PS3. NAT Type 2 is basically where the hosts on the private (home) network can get ports associated with them.
NAT-PMP[]
NAT-PMP is a Network Address Translation Protocol addition promoted by Apple Computer that competes with the less secure UPnP networking protocols. The Apple Airport Extreme router uses NAT-PMP as do some other Apple networking products. This NAT port mapping protocol should provide seamless association of ports and machines, but seems to run into problems when there are multiple gaming machines in the private network trying to join or host games.
Multiple PS3 machines in one NAT network[]
If you have multiple PS3 machine in one NAT shared IP address network it seems best to have one of them act as the host. If port mappings in a type 2 NAT network get mixed associations none of the PS3 machines will be able to act as a host. After restarting your router, start only one PS3, and Host a game. Then start the other PS3s. The one host machine should be able to act as a host, and the others should be able to join.
Controller ID (PS3)[]
Avoid using your controller as number "2". This can lead to invites from friends not appearing and other bugs on joining multiplayer games, such as your microphone not working.