Use the S/PDIF ports (in & out) on the back of the MK3. Get a male_to_male RCA cable and jumper these connections. You then configure MOTU's Cuemix software to create a bus that sends the signal from the S/PDIF Input channel to whatever Output channels you desire. The idea is you are taking the output signal from your computer (USB --> S/PDIF Out) and sending back into the MK3 (S/PDIF In) so Cuemix can see it as a valid input and allow you to direct it to as many outputs as you want. This is akin to a recording studio sending the lead vocal (i.e. from the front microphone input) to the various other musicians via the output ports.
Direct for your computer's "on board" sound to the ASIO S/PDIF (output) port. If memory serves, I thinks its outputs 11 & 12. Of course this step could be done before or after the Cuemix adjustments.
Danny B.