Hi jchambers:
From this page:
http://www.asus.com/Motherboards/AMD_AM3Plus/SABERTOOTH_990FX/#specificationsSlots 3 x PCIe 2.0 x16 (dual x16 or x16, x8, x8) *1
1 x PCIe 2.0 x16 (x4 mode, black)
1 x PCIe 2.0 x1
1 x PCI
This means the top 16x slot, 3rd 16x slot and bottom 16x slot are the three to use in 3-way SLI and CrossFire. The board supports both types of multi-GPU with the latest graphics drivers. Keep in mind if you're running 3-way with dual-slot GPUs the bottom slot requires extra space in the case for the graphics card to overhang.
The 2nd 16x slot is the only one that's not suitable for graphics at 4x.