Not sure about three but this is what I did for two.
The ports are 53, 80, 88, and 3074. For two Xbox consoles use those ports and since you are trying to connect to a server, what you need to do is to use Port Range Triggering. You can't use Port Range Forwarding since it will only set those ports into listening mode to the IP address you set it. So if you use Port Range Forwarding it will only be open/available to one console(the one using the IP address).
You need to use PORT RANGE TRIGGERING. Disable Port Range Forwarding and DMZ. You need to enable UPnP as well if your Linksys router has this option (some other models don't have this option but it is said to be enabled in default settings according to their tech support).
To solve the lag problem set your MTU size to 1364. This settings will work even if you have one or multiple consoles running behind the router.
if you need more help let me know. I will try.
