The IDE interface itself can only accept 2 drives per 'channel.'
There is probably another connector on the motherboard similar to the one that the hard drive ribbon cable plugs into. The cable from this one may plug into your CD-ROM or CD-Burner and should also have two connectors. You can put the third hard drive on this second cable, just make sure to set the hard drive as 'master' and the CD-ROM as 'slave' using the jumpers on the back of each device. If speed is a concern, plugging a hard drive into the same cable as a CD-ROM drive may slow down that hard drive, but there is no problem using this configuration.
If all four of these connectors are used on both cables, you may be out of luck. You can purchase a seperate card that plugs into the white PCI expansion slots (a 'PCI IDE controller'). These host extra IDE channels that you could then plug the third hard drive into.