have you tried sapphire trixx ? works for me with 2 GPUs, but I don't have a 3rd to try it out for you.
Tried it just now with no luck. I will illustrate on screenshots:
http://img40.imageshack.us/img40/7906/trixx1.jpg
http://img560.imageshack.us/img560/5017/trixx2.jpg
So even though Trixx shows 960/300, the reality is 960/1450
