Please refer to page 79 (Chapter 5) of the manual: http://download.gigabyte.us/FileList/Manual/mb_manual_ga-890fxa_ud5_v2.0_e.pdf
From the manual:
- Make sure OnChip SATA Controller is enabled
- Set OnChip SATA Type to RAID
- Enable ports 4 & 5 if you need them.
- Save the settings.
Then reboot and follow the procedure on page 81. At POST, press <Ctrl> + <F> to enter the RAID BIOS setup utility, select RAID 0, assign all 3 drives, etc.
Since you're setting up a new system, don't be afraid to make changes in the BIOS. If something goes wrong, you can always reset it to its default values.