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:
title Red Hat XXX
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.