Unfortunately you cant use the Belkin Pre-n router with a USB ADSL Modem. You will need to get a combined ADSL Router and discard the modem. When you set-up it will detect your ADSL settings automatically, you just need to input your username/password etc.
If networking 3 PCs and they aren't going to be near each other, then you will need a wireless ADSL Router. You connect one PC to the router via cable, then wirelessly connect the other 2. If the other 2 PCs dont have wireless capability then you will need to purchase wireless cards for them. When all 3 are connected you run the Windows Networking Wizard and off you go....
Have a look at this website
HERE, it gives good guidance including a Wizard to help you select your equipment.