it all depends what boot loader your using , you gotta be more specific , grub ? lilo ? or something else maybe ?
install in order from oldest to newest.
What operating systems? And what boot loader? GRUB? LILO? Actually, you may want to try editing the boot.ini file by clicking startrunmsconfigboot.ini You'll see the line operating system, and under it should show up at least one OS. Just add the other two!
