I have several rigs using this board and I can tell that every board behaves different.
After installing my first rig using 5 R9 280X, 1 Corsair 1000W and 1 750W psu, I assumed that I could easily finish the rest because they're identical.
But unfortunately not.
First rig with normal setup with power to the board. Second rig needed power from both psu, each psu to the board.
Third won't work unless there is no molex to the board!
4 rigs up and running each setup different and now I am facing some different issue on no.5 and still can't resolve it. Nothing seems logical.

So in short, it is a pain in the ass to get it all working with this motherboard. I suspect the motherboard is to blame.
I never had this problem when I used Gigabyte motherboard, but they are more than twice as expensive.
But don't buy Gigabyte because they won't work with BAMT. There is a solution, but that's another story.

Try different setups using different pci-e ports, try using 1 molex to the board, try none, etc. Some combinations just won't work.
Which psu do you got? Is it a single rail? Go through everything and try some crazy illogical stuff too.
You should be able to plug 4 R9 270X on your psu with undervoltage. 3 cards should be under 900W.

Good luck!