aplay -D plughw:0,3 /usr/share/sounds/alsa/Front_Center.wav plays sound
-D = device
plughw = output to hardware (and resample based on DAC)
0,3 = Card 0, Device 3
I would expect that board to only work as hw:0
As your sound is now working, if you cat /etc/asound.conf I expect you will see that is what is being output to.
In myth, you will need to go to the frontend (not backend) setup. Go to General... after a couple of menus you will see Audio. Move it from "Default" to the hw:0,3 and change it to hw:0.
For some reason backspace in myth is interpreted as a "go back one menu"... so use delete. I am working on fixing this default problem in myth.