Well on my old board, everything I listened to output to all of my speakers. So I'm not sure if its that. I tried what you said and everythings fine, but it still only outputs to the rear left/right speakers unless I click on "speaker fill" in options and then it lets me use the rears and the center. But the two other ones still don't work.
I noticed that when I plug them into the onboard audio, that I can plug any one of the plugs in and whichever I plug in the speakers corresponding to that plug will work. Which means that its not detecting all 3 plugs at once, but a single one. Whichever is in the output slot.