I've edited the initial post to stress the rig needs to be restarted.
How do you launch the program so that it uses your 2nd or 3rd video card instead of the first?
I never got that part figured out.
AMDOverdriveCtrl [-i {3,6,9,etc.}], where -i is usually a multiple of 3. You can find the right index by doing:
AMDOverdriveCtrl -h
and using the index numbers that are active.