I'm assuming those computers can't sit in the same room and have cables between them. Let me guess, at least one of them is a laptop and will move around.
You could get a wireless router and make sure at least two of the computers have wifi (buy add-on PCI-E cards if their motherboards don't have it already).
Edit: contact your ISP about this. They know what kind of modem you have and what works best with their service. They can recommend a router. Expect to pay $100 to $200 for a good wireless router and maybe another $20 to $50 for each wireless card if needed. Configuring the router can be tricky, and the ISP should help you with that. Well, mine did, at least...