Welcome to Tom's Forum!
Huh..."In addition, GIGABYTE DualBIOS™ now supports 3TB+ (terabyte) hard drive booting without the need for partitioning, and enables more data storage on a single hard drive."
... RAID 1 is as simple as it gets mirroring?!
ref - http://www.gigabyte.us/products/product-page.aspx?pid=3927#ov
You need to set the RAID in the BIOS. I assume both 3TB HDD are connected to the AMD A75 SATA3_0 & SATA3_1 ports.THIS WILL ERASE EVERYTHING ON THE HDDS!!! BACK-UP FIRST!
OnChip SATA Controller -> Enabled
OnChip SATA Type -> RAID
Save & Exit = Yes
Press <Ctrl> + <F>
once in the RAID menu
<Ctrl+C> ; to define RAID
RAID Mode ; <Space> till RAID 1 shows up
Drives Assignments ; <Arrow> to assign HDD and <Y> to select
/Do for both drives/
<Ctrl>+<Y> ; Name e.g. RAID
<Ctrl>+<Y> ; Erase and Create Array
<Enter> ; Maximum capacity
<Esc> ; Main Menu
Press <1> to View Drive Assignments
<Esc> and reboot then install the OS.