Math doesn't add up linearly. Any of the 580's can hit @ 220-240w each, and a 5700xt can hit over 280w by itself.
So on the low end, you are looking at 940w just in gpus, with another 100w in cpu and 100w in everything else. So a total possible of @ 1150w.
Your psu is underpowered, the only reason you aren't hitting its max is pure lack of use per card. If you did anything other than mine, which is possible since Windows 10 is multiple gpu based (DX12) and not sli/CF based (DX11), you'd most likely trip psu protections, shutting down the pc.
It's doubtful all the cards are running equal, considering their power draws, more likely is the 580's are pulling about 140w each and the 5700xt is pulling closer to 180w.