I have done some digging around on various forums so this advice is just what i have gleaned from others and not my own knowledge. Check in your motherboard manual and check in the bios to see if the setting needed is there first.
So what did i find ? Well apparantly the PCI-E configuration on your board is 2x PCI-E 2.0 and 1x PCI-E 1.0. now reviews i have seen put them in differant orders one says the 1.0 slot is the third slot and one says its the middle slot and you get an extra long bridge to get full2.0 SLI. So as i said check your MoBo manual to see what you have.
Secondly a couple of guys on forums say you can change your primary slot by reasigning it in the Bios for SLI they are saying to change it from peg to peg 2, so im guessing you should have a peg 3 option.
As i said check that its all there first and then i see no reason not to do it. Or you could google about the peg thing if you are still not sure.