I don't believe it's possible to have 2 sticks running in dual channel mode and another single stick not running in dual channel mode. If you try that, dual channel won't work. If you want 3GB of RAM to work in dual channel, you've got to get another 2 sticks of 512MB and put them in the other 2 slots.

So it's either everything in dual channel mode or nothing, as far as I know.