|A floppy drive is not IDE but that's just an aside.|
For multiple entries in device manager the first thing to try is to delete both entries and then on reboot let windows set them up again. Another option, if you have no IDE drives connected, is to disable that controller in bios setup.
You apparently want to have three SATA drives. That shouldn't be a problem as long as you have three SATA motherboard connectors. If you don't then you'll have to add a SATA PCI card or have the third drive connected externally via USB. Are you sure the other Dells have three SATA drives or could at least one of the drives be IDE?
You mention what regedit, explorer and disk management see but what really matters is what the bios sees. If the bios sees a drive then windows will see it too.