You should not need an external enclosure. What you DO need, though in your machine is:
1. An open physical slot in which to mount the 3rd HDD;
2. An unused HDD port (I presume SATA, unless the Samsung is an older IDE unit) and a data cable to connect from HDD to mobo port;
3. An unused power supply connector on the wires from your PSU, suitable for your HDD (SATA or IDE).
With all the RAID controller systems I've seen, even though your SATA or IDE ports may be configured for RAID use, the actual RAID software does NOT use any HDD as part of a RAID array unless you specifically tell it to do so in the RAID configuration screens. Any HDD not assigned by you to be part of a particular RAID array remains a NON-RAID unit that you use as a stand-alone drive. So, once you mount and connect the Samsung unit inside and reboot, it should show up in My Computer as a new drive ready to use. (I am presuming that is already has been Partitioned and Formatted, and contains some data you want to keep and use.)