Yes.. technically you can connect this third hard drive in lieu of your cd-rom drive, ASSUMING it's an IDE drive (uses the same connector).
If you're wanting to hook it up temporarily don't even bother mounting it. Just shutdown, unplug the CDROM, make sure the jumper setting is the same on the hard drive you're putting in (master vs. slave), plug in the hard drive... just kind of let it hang. Boot up.. copy it all over to one of your other two hard drives... unhook it and then plug the cdrom back in.
If you want to hook it up permanently, and you don't have an extra hard drive bay, AND the cable won't reach.. then you can purchase a 5 1/4" to 3 1/2" bay adapter. The hard drive mounts into the adapter, and then the adapter mounts where the CDROM currently is. They even make some with fans built in, but I doubt you'll really need that being it doesn't sound like a real high performance setup (but I could be wrong).
ANOTHER thing you can do is purchase a USB external enclosure.. usually like $30 or so... you can mount the hard drive in that.. then connect it to your computer via USB.. then you won't even have to bother taking your computer apart.
Good luck