Hey, firstly, if you run the SSD and HDD in RAID 0, you'll get the same capacity as two of the SSDs, example, if your SSD is 240GB your array will be roughly 480GB. Another thing to note with RAID 0 is that if you lose a drive to corruption, or it dies, you lose your information. If you want RAID 0, I'd get another 3TB HDD and chuck both of them in RAID 0. otherwise, you could go for a RAID 1 (mirrored data for backup) or RAID 5 (like raid 0 but with some chance of recovering data).
Hope this helps.