You never want to mix HDDs and SSDs in a RAID array. The RAID array should be all HDDs or all SSDs.
A RAID array's Read/Write performance is based upon the drive in the array with the slowest Read/Write speeds.
So you would be wasting your SSD's performance if you put it in a RAID with a HDD.
A RAID-0 array's total capacity is based upon the drive in the array with the smallest capacity.
So if you have a RAID-0 array with 2 1TB drives and 1 120GB drive your array's total capacity (before formatting) would be 360GB (120GB x 3).
If you're determined to create a RAID-0 array then create one with just your 2 1TB drives.
Install your O/S on your SSD.
Maybe you mis-understood. 2 1tb HD are RAID 0
the 120SSD is for OS and other things that needs quick booting