It is not so simple.
IIRC, if you put 3 VGA's (Tri-SLI) in slots 1, 2 and 3, you will have following configuration:
slot#1 @ 16x
slot#2 @ 8x
slot#3 @ 8x

not x8, x8, x16 that might be expected and what diagram shows. It has been argued before, asked Asus, and they said it's like that.
IIRC.