I just got my 3X Sapphire Dual X R9 280X OC cards stable after alot of tinkering. I still haven’t tried to tweak cgminer for better results, just getting them stable was a chore.
I also have 2 msi 7950 reference cooler cards there as well.
I had planned for a 6 GPU setup, but 2 of the powered risers i bought from ebay did not function right and caused all sorts of lockups.
I had 2 corsair RM 750 watt power supplies using a 2x PSU adapter. Worked well, once I got the cards running full speed the 3X 280X cards were drawing 850 watts at stock volts. So i switched it up to run 1 7950 and 1 280x with the main-board/components and 1 PSU to run 1 7950 and 2 280X cards. Now the system is stable.
In conclusion, i would definitely use a 850-1000 watt PSU per 3 280X cards. Make sure it is gold plus or platinum for efficiency and preferably a brand with good reviews and rated to run 24/7 at 80-90% loads.