Yes, previous answer is quite right.
Windows 98 or 2000 will allow ICS - Internet Conection Sharing. This means one PC is connected to the Internet via it's noral dial up connectuion. The other 2 are connected to the first via an Ethernet network. You'll need an Ethernet card in each PC (about �15-20 each) ,a small 5 port hub top connect them together (about �35) and some Cat-15 RJ-45 Cables to connect them up together (about �5 each). If you go to Windows Help and search for Internet Conection Sharing you'll find a step by step wizard to help you get going - it's not tricky so have a go.
As previous responder said, the speed of your modem will be shared so if you're downloading files on all three it will slow down a bit. Also, the Ethernet network will mean you can share files and printers between the pc'S