Yes, it can be done. That is the exact setup that I'm using:
2x 160GB Seagate 7200.7 RAID 0 on SATA1 and SATA2 (red ports)
1x 320GB Seagate 7200.10 single disk on SATA3 (black port)
The mobo manual says that the RED SATA ports are for boot disks and the BLACK ports are for data only. So, connect your 2 drives to RED ports and the backup drive on a black port, setup the RAID 0 for the two drives and you should be good to go. Follow my example above and you'll be fine.