You may have been given the wrong info or have it mixed up because there is no mac address from Comcast but you do have an Ip address from them. You can clone the mac address of the router , though I don't know if that will help. You do want to make sure that your ps3's NAT is open.
Do you have any other devices connected to the router ? Laptop , computer , smart phone or tablet ? Does your router stay connected to the internet or is it just the Ps3 that can't stay connected ?