Well the 850 watt PSU exploded. The 750 PSU powered 2 of them fine.
I purchased a 1000 watt PSU and it has been running all 3 gpus overclocked for a week solid! Though thanks to this ridiculous difficulty factor increase this rig I built is not even close to being profitable.
It was worth a shot...
And that kids is why you buy decent power supplies.
Right. Two Gpu's, 850W minimum. Three Gpu's, 1200W.
My rig with a 5850 and a 5870 consume ~550w at full tilt. That is with multiple hard drives and a power hog of a CPU. That power supply was just shit because his load should have been somewhere around 750-800w.
Also make sure to run amd cool and quiet and set the affinity for the miners all to one core. It will underclock 3 of the cores and bump about 50w off your consumption.