You cannot add a third hdd to a RAID 0. You can reconfigure the RAID to RAID 1 though if your RAID Controller supports - but that would mean destroying existing RAID 0 (and losing all data) and configuring RAID 1.
You could setup the third HDD as a Simple Volume in your RAID Controller, after which it should show up in computer management. But be aware, this HDD isnt in any RAID config, its just configured as a simple volume.(no redundancy for data on this drive)
SC