Currently I have a dual boot system with Windows on one HD and SuSe 9.1 Pro on another. Both drives are on the Primary IDE channel with the Windows drive as master and SuSe drive as slave. GRUB is installed on the MBR of the Windows drive.
I would like to try out another Linux distro on a third hard drive and get a triple boot option using GRUB if that is possible. On the Second IDE channel is the CD-RW drive as master. Could I put the third HD on as slave and then install Linux (may try Yoper) and choose not to install a boot loader and then modify GRUB to reflect the additional HD with Linux installed?

I don't know if you have that 3rd HD already or still intend to buy it but aren't they large enough to use partitions instead of putting one distro on one HD. Things like your /home/ folder can be used by both distros.

You can modify GRUB to see that 3rd OS but I don't know how cause I use lilo.

In theory, yes. In practical use, you can trash the .kde or .gnome configuration if the versions and/or installed packages conflict, and the session manager can be totally screwed by sharing ~.