Install Windows 98 to the third drive.
You will now have two bootable drives. If you want to use the BIOS to boot into one or the other operating system. Set the BIOS to boot from the drive which has the operating you want to use.
If you want the boot loader that XP gives, to give you choice of operating system.
In XP go to start > Run > Copy and Paste:
Click ok... Below the line of "multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Home Edition" /fastdetect"
Add this line underneath:
Reboot XP and you should be given a Menu. Select 98 and see if she boots.