You would need something like a USB Capture device and insert it in between the ps3 and the tv and connect it to your laptop with the usb cable.
This, of course, means your laptop would be close enough for you to just watch the tv instead - rendering the whole setup as pointless IMO.
If you meant, how do you watch tv on your laptop while the tv is being used with the ps3 then you would need to purchase a usb or network tuner to use with your laptop. A USB tuner would need an antenna or cable to be logged into it and then its connected to your laptop. A network tuner also need a cable or antenna plugged into it and it plugs into your router. From there you can access it wired or wirelessly. Please note that neither device would let you watch encrypted channels.