|Replace the router with at least 5-port switch or hub to get IP's directly from your ISP. The router takes one IP address and splits that into private addresses. |
Alternatively, you could get 3 of those machines assigned with your router - turn OFF DHCP in the router and plug up to 3 computers + modem into the 4 PC ports, leaving the Internet/WAN port unplugged. That will turn the router into a switch with wireless capabilities for the remaining IP address(es). If this is too complex, just go with the switch/hub solution.