I've been browsing the web for some answer to this question for some time now, but it seems as people can't decide if it does or does not support the three way SLI, that would in my case mean 3x Gainward GTX 670.
Ok , here are your choices and they are limited to what the board has for slots and what you are going to use for video cards.
The product page on the Asus site says that this board will support two way , three way and quad SLI. The problem comes when you realize that they are actuallly talking about using dual gpu video cards as well as single gpu video cards.
So for two way SLI you can use two single gpu video cards in the first two full sized slots and those slots will be running at x8 bandwidth.
For three way SLI you can do two different setups.
Three single gpu video cards in the three full sized slots with the slots running at x8 , x8 and x4 and it will work that way with the third slot running at a x4 bandwith.
2. Use a dual gpu video card ( gtyx590/690) in the first slot and a single gpu video card in the second slot.
Finally the quad SLI setup would be with two dual gpus (gtx 590/690) in the first two slots running at x8 , x8.
You could also run quad SLI with a dual gpu and two single gpu cards in the three full sized slots running at x8, x8 andd x4 bandwith.
So in your case you can run the three way SLI by using the three full sized pci-e slots and they would be running at x8 , x8 and x4.
The problem that you run into with the Sandy Bridge and the Ivy Bridge is the bandwith lanes that are avialable to the PCI-E slots-- 16. These lanes are devide between the slots depending on whether or not something is put into the slot. When just one video card is put into the first slot then it can have all 16 lanes, when a second card is put into slot 2 then the lanes are devided between the two slots 8 for each one. When you put a third card in the third slot then your getting 4 lanes from someplace else to use for that slot.
If you already have the motherboard and cpu then your locked into trying to make the best out of it. If this is something your planning to build then you do have an option and that would be to go with the Asus Sabertooth LGA 2011 socket with the X79 chipset and it's massive 40 lanes of bandwith.
The video cards that are currently out today will not saturate the Pci-e 2.0 slots so when it comes to the x4 slot you will see about a 25% loss of performance from the card. Before doing that though I would try to find out what happens when a third card is put in that slot and where the MB gets that extra 4 lanes from.
The lucid Logic is a process where if you have a video card in the first slot you can combinr the intrgrated graphics with the video card for a 60% increase in performance. So maybe they have a way to use the Lucid Logic chip to get that extra 4 lanes.
You can go on the Asus site and send a question to the tech support with the exact question you want answered about that third slot. Since you have the board already then in the owners manual it should give you a layout of what the slots will be running at when you put video cards in all three slots.