when you say it is not working can you elaborate a bit more on that. With regards to dual channel you can not just turn it off, you just don?t occupy any DIMMs in the second or first channel if you only wish to utilize one channel, the different channels are usually colour coded to stipulate the differences .. look in your manual for channel 0 and channel 1, and what you should channel you should occupy for different configs..
You should have no problem using three sticks on that mother board, as Anubis rightly said, if you use more then two sticks of double sided DDR, the memory controllers will default to DDR333 PC2700, and that is for all of the memory as they do not run independent of each other. None the less they should work, you might however have a faulty stick in one of the channels that are causing you the problems, once again if you can describe the problem it might be easier to diagnose .. as a pointer if you can, try and boot into memtest x86, and run a few passes within that to see if it fails any test (tests 0 - 9), this is a good diagnostic tool, i would also try and test each stick separately to try and single out a possible faulty stick.
