Are the computers all set to 'Obtain an IP address automatically' and 'Obtain DNS server address automatically'? If yes, do you know how to access the page on the router/modem that lists the DHCP leases? Check to make sure that there is only 3 computers listed and none of them are listed twice.

Two other things I'd like you to do.

1) Make a ASCII drawing of the network setup in your house starting from the internet.

2) What is the name and model number of your modem and router?