I am a little confused about your arrangement. For a home setup with three computers, the cable modem would connect to the WAN port of the router, and the the three computers would connect to the LAN ports. If this isnt how you have them connected, please change the config, and in particular if you have ICS running, take it off.
To connect a Win XP (Home?) pc to an existing network, please see
www.practicallynetworked.com for troubleshooting tips. First, the built in fire wall must be disabled, two - use simple file sharing, three do not run the network wizard, repeat do not run the network wizard on this or any other machine. To do all this you must be logged on as an user with admin rights - you create such an account as soon as you start the XP PC for the first time. First get the network going, then connect to the internet. You may wish to add Netbuei as a second protocol in all the machines - please MSFT KB article 301041- and then fine tune the network to maximize security. Go to dslreports.com forums on SMC routers to check for the optimum port settings. Also do a TCP/IP analysis using their tools to tweak your registry settings.
Important thing to keep in mind is that the XP pc must adapt to the other two PCs networked not the other way around.