a) The floppy you are booting from may contain a HDD driver that can 'see' large HDDs
b) Why do you boot from floppy?
Why don't you just add Slackware and FC3 to the list of options for lilo?
Here is an example
Even better. I would recommend that you let FC3 install grub during installation
Then edit the /boot/grub/grub.conf file to add your other two distributions.
(just make sure that you knwo where the kernel image is in each one)
Here is a FAQ
Once you do that you'll wonder why didnt' you do it a long time ago.