Leave your SATA ports set to RAID in the BIOS, you just don't set up the 3rd drive in RAID, since it's not!
RAID is a version of ACHI, or visa-versa.
If you don't set up a(ny) drives in RAID, they won't be RAID'd.
I used to have 2 SSD's in RAID 0 for OS, and then had 2 HDD for Data/Media and BackUps, each separate, so the OS "saw" 3 drives (C:=OS, D:=Data & Media, E:=BackUp Drive).