I was trying to get you to test the slots really, not the memory itself. You can use 6GB / 3 modules if you want, please test as I mentioned previously, thanks!
As for voltage you may need to increase the QPI/Vtt voltage, the ram voltage should be fine at it's stock spec'd value. You need to do the test I mentioned above before anything else though, so you can see if you have bent CPU socket pins. Or you can remove the CPU and look if you want, if you do that please take several images for us to look at. Do this in a well lit room with no flash, and the camera set to macro mode if it has that option. Take the images from several angles off to the side looking into the CPU socket, just not directly over it.
And be VERY Careful while there is no CPU in the socket!!!