I've just added a 3rd R9 290 to my rig and it's powered with a separate psu.  I first tried using just a normal powered riser, but as soon as cgminer started the pc would crash.  Tried all different wiring configuration, but always the same results. 

I ended up using one of the USB risers which lets you totally separate the GPU power supply as the only link to the motherboard is data.  Plug it and worked first time.

Only down side is there twice the price,  But it I had got this from day 1, I would have gained 4 days mining, sound would have paid for itself.

Hope this helps