Yes you are coorect. all you need is the Wireless Ethernet Router that supports cable. Make a note of the MAC address of the router.
Plug the router in your ethernet port and into the either the cble point on the NTL box or the Modem. Make sure that no usb cable is going from the modem to the PC ( if the Modem is a USB Modem). Oce that is is done, see if you can access the interent. if all is well, then the other two machines will be able to see the router and share a connection.
if the above doesn not work, you need to phone NTL and tell them to change the MAC address from the existing modem to the MAC address of your router. This may take into effects from minutes to 24 hours.
Let us know how it goes