I need help with my Dell T5500 Rig, I'm trying to set up 3 RX580 GPU on it, I ran into problem when trying to power up all 3 of the RX580s, I bought the "8 pin CPU to 6+2 PCI Adapter" from Amazon for the third GPU, however when I connect the third GPU, DELL T5500 does not want to power (the power LED flashes orange a few times and it powers off). Why is the CPU power not powering the GPU using the adapter?

I have no problem running 2 of the RX580s, they are power by 2 of the 6 PIN PCI Cable using 6PIN TO 8PIN adapter.