The SB X-Fi specs says that is PCI-E 1x, so, should work in the black or both white lines of ur mobo.
Ohhh righ like ASUS support:
Thanks for contact with ASUS support,
The SB X-Fi that u want buy is PCI-E x1 and the Crosshair III have 3 slots that meets that requirements,so u won't have any problem installing that card in the mobo.
Thanks for contacting with the ASUS support and have a nice day.