To get the XB3 installed to replace your router, call customer service and ask them to schedule a tech to come out and replace it.
As far as trying to replace the routers with XB3 gateways, technicians that go to houses for an Xfinity Home service call and see that there is a router will usually swap it, because of the issues the routers can have. Not everyone notices if they do have issues with the routers either, it all comes down to what they have installed and how much they interact with the system. Generally people with cameras or people that use the app will notice the router issues, but if you are only using the touchscreen or the keypad, you may not ever notice any connectivity issues. (the system still functions as far as calling out on alarms though because of the cellular.)