1) You need other small HDD like 250GB at least. Then cloning the 120GB SSD ( whole drive) into the small HDD. After cloning boot the PC using the HDD to make sure it works before you set up the RAID because all the data and files will be gone. And install the Intel® RST RAID driver if you use the intel system,
https://downloadcenter.intel.com/Detail_Desc.aspx?DwnldID=22194 or AMD raid driver
http://support.amd.com/en-us/download/chipset?os=Windows+7+-+32 for the AMD PC.
2) Connect the two SSD into SATA_0 and SATA_1 port, the HDD in other SATA port. Boot the PC and go to the BIOS to set the storage mode to RAID, also set the boot order is HDD=1st. Saving and exit. Reboot the PC and setup the raid0, check the manual to see how.
* Make sure don't make mistake to raid with the HDD. Double check.
3) Boot the PC using the HDD, then recloning the whole drive back to the SSD RAID0.
If you have the WD HDD, using Acronis True Image WD Edition Software
http://support.wd.com/product/downloaddetail.asp?swid=119&wdc_lang=en
For Seagate HDD, using DiscWizard http://www.seagate.com/support/downloads/discwizard/
Or other software comes with the SSD.