If you are using Fedora, then you are using the GRUB bootloader. After you install the third OS, you will need to make an entry in your GRUB configuration file to give you the choice at boot time of that OS. The files you will need to edit are in /boot/grub/grub.conf. If you will study this file, it will give you an idea of how to add the entry for the new OS. Also, google for a grub tutorial, there are tons of tutorials about grub on the net.
Hope this helps