You'll definitely want to do a couple things. First, you'll want to assign static IP addresses to both PS3s. Once you have done that, you'll want to place one of the PS3s in the DMZ of the router. For the other PS3, you'll need to forward the appropriate ports. The following info should help:

1. http://portforward.com/networking/staticip-ps3-playstation-3.htm This guide will show you how to set a static IP address for your Playstation 3.

2. http://www.portforward.com On this webpage click on Router List > Select your router > Select your game (PS3 Call of Duty: Modern Warfare 3) > Follow the steps in the provided guide.

3. Disable any firewall or security features within your router.

4. Set port forwarding on your router to the IP address of your Playstation 3. This game uses ports 81, 3074, 3075, 3478, 3479 (UDP) and ports 80, 443, 3074, 5223 (TCP).

For voice / video chat and online game play, direct communication with other PS3 systems is used for data transmission during voice / video chat and online gaming. The port number used for this is UDP: 3658

Once you have done that, go ahead and restart your console, modem, and router. Try to connect to MW3 once everything has been restarted.

When you have multiple consoles playing behind the same router/modem on the same network, this can cause these sorts of problems.  In my personal experience, when it comes to having two Playstation 3's behind the same router, you will want to assign separate Static IP's to both consoles, forward the ports for one, and set the other as the DMZ host.

You mentioned that you are using a modem but you have multiple consoles connected.  Are you using a router or a switch?  Please provide us the make and model of the device so we can see if there is anything else that can be done as far as troubleshooting this issue.

Xbox has the same probs it all depends on the router (and its settings of course).

I´ve had te same problems for a very long time (sometimes it worked sometimes not) and it all changed when I recieved my new router.

I´ve tried for months to find a solution in the end I could say that some routers support settings that allows you to play with multiple connections and some don´t.