The cable modem has nothing to do with this its JUST a bridge, a blind, dumb, bridge, unless it has a router inside it and even then its not a commercial class router, so its not going to help.
I am not sure why you would want to do this, but what you need to do is set up a proxy SERVER and then set the setting of the other two laptops to use its ip as a proxy device, that or use it as their internet connection and do the proxy behind the scenes.
Your best bet would just be buy a router that supports whatever you want in the proxy and use it, but whatever.