here's the way I'd set it up (no guarantee this will work for you)....
Read the router manual...if you don't have one, I think they can be downloaded from Linksys.
Plug the modem into the USB port on the router (may need some additional configuration...I've never used a USB modem).
Plug the 3 PCs into 3 of the standard router ports.
Fire everything up.
Open a browser and enter http://192.168.1.1
....if you get a login prompt, that's good. Enter nothing in username, and "admin" as password, then you can configure your connections if necessary.
If you don't get a login, then you don't have DHCP client running properly on that machine.
This isn't a full description, as there are different ways to do it....this is just the simplest IMO, using dynamic internal IPs. As I said, though, I don't know how USB modems differ from what I have.
Also, f you are currently using static local IPs, this will mess up your current setup.