When you install openSUSE 10.3, you can choose to install GRUB - a boot loader (where you'll be able to choose the os).
I would choose to install GRUB on your third HD.
openSUSE's installation process will take care of the configuration for you. I know it detects XP and probably 2003 too.
Whatever you do, I do *not* recommend using microsoft's boot loader.
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