First locate your grub configuration file. For example, /boot/grub/grub.conf, open the file in a text editor and you should be able to see something like this:
default=1
timeout=10
....
title Red Hat XXX
root (hd0, 0)
...
title XP
root(hd0,0)
...
title <your third OS> <---- Is it in your grub.conf ???
and etc. If you want to choose to boot from your installed 3 different OS, make sure it is included in your grub config file.
Hope it helps.
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