be advised that you can run a mixed ethernet and wireless network - I do it myself.
The way to get the 98SE machine correctly networked is to make a floppy using the networking wizard on the XP machine. Run the disk on the 98SE machine (using the setup icon), and of course make sure that the workgroup name is identical on all three computers.
Set your permissions correctly on all machines, sharing the folders that you want to be available on the network. If you're running a firewall on any of the computers you may find that it stops access, and you'll need to configure it accordingly. Get the network running by disabling the firewall; you can configure it afterwards, when you've identified it as the cause of the problem.