...as raid or straight IDE? If as a plain IDE channel, consult the users manual. This is a function of the bios or a jumper on the board. Some motherboards allow you to use the third ide connector as a plain ide drive.
 
I remember trying this with an MSI board, that allowed IDE 3 (raid 1) to be used as a plain IDE channel. Mandrake saw the drive as hde, but also saw the hde drive as the primary drive on detection. I was using lilo at the time and had to tell lilo to reverse the drive designation. Since Mandrake saw thew first raid connection as the primary drive, I passed the ide=reverse command to lilo in the Mandrake boot options. There is also a way to do this in Grub, if this is what you are using as your bootloader.
 
If you already have a hard drive in the #3 connector, go to a terminal window, as root user and type in fdisk -l (that is the letter l, not the number 1) and see what the designations are. In my case, it was the following;
 
hda = primary master
hdb = primary slave
hdc = cdrom drive (secondary Master)
hdd = secondary slave drive
hde = drive connected to the third ide connector (raid)