I got 3 cards to work with the following (I just use it for playback, great for mixxx):
Based on http://www.sound-man.co.uk/linuxaudio/ice1712multi.html but adapted for my setup and only playback.
# First 6 channels of first soundcard (playback)
# First 2 channels of second soundcard (playback)
# First 2 channels of third soundcard (playback)
I then use jack with the following .jackdrc:
I use 2 cheap usb sound cards and choose the outputs 7-8 and 9-10 on mixxx for master and monitor.
/usr/bin/jackd -R -m -dalsa -P -dmulti_playback -r48000 -p1024 -n2 -m
It also works well on my setup for playing around with audio production (seq24,hydrogen,nekobee,zynaddsubfx,jamin,jackeq,p atchage). On the Macbook the "line as output" option doesn't work since the line is muted when using the headphone jack, and I couldn't use it for monitor.