Wait a minute. I didn't see you only have a 850W power supply.
I killed my 550W power supply running a bare minimum system on one 7950. I had a 620W power supply spare and that solved the problem. My card takes 2 PCI-E connectors at the top, both were connected. My "symptoms" were:
- I could boot and run the system fine
- I could mine for about 5 seconds
- I used the "sensors" tool that comes with linux that monitors voltages and I could see right before it failed that my 12V would drop to 10 V and then the system died
According to this page
http://www.anandtech.com/show/5476/amd-radeon-7950-review/16
under load the card needs 353 Watts. So running the 3 cards alone (no motherboard and no CPU) you already need 1059 W! I'm not an expert with running 3 cards, I have never seen someone suggest such a huge CPU. Would be nice if someone can correct me.