What are you trying to accomplish here? File sharing? Linux box as firewall and filesharing? What have you done so far?
For filesharing you will need to configure SAMBA. Are running FC 6 in GUI mode if so which one? Gnome or KDE? I run FC5 and it defaults to Gnome so if you installed using defaults then it probably is gnome.
Work out exactly what you would like to happen between these three computers and post it here so that people know exactly how to help you.
In any case since you want them all networked, make sure that all three boxes are on the subnet mask i.e. if you use static ip addressing make sure that they all match what the linksys router is on. In most cases this would be ip: 192.168.1.X (x - unique no for each machine) subnet: 255.255.255.0 and gateway: 192.168.1.1 (i am assuming this is the ip of the linksys router) and DNS: 192.168.1.1 (again assuming linksys router has this ip address).
If you would like to use file sharing between the linux and other xp boxes then SAMBA will need to be configured but let us know what you want to accomplish.
Ignore my comments about network ip addresse's if you are using DHCP to get IP's from the linksys router.