Getting desperate as I can't find any answers on the interwebz.

Hoping someone can give me a few ideas

So I have a fairly basic setup with 3x 1200 watt server PSU's, Intel Celeron G3900, 8GB of DDR4 ram and 120gig SSD running on windows (just while I get setup).

I have a mix of GPU's at the moment and I've been following the mining guide from Asrock to the letter.

Added the first GPU (1660 Super) and everything booted up fine, added the 2nd GPU (3070) everything booted fine and was detected, added a 3rd GPU and suddenly the board just doesn't boot. Black screen/no bios.

If I take out the 3rd GPU everything works fine, but I've tried 3060's, 3060 ti's, 5500 XTs and 5700 XTs and the problem keeps happening. As soon as I add that third GPU everything stops.

A few others things I've tried

Replaced the riser
Put the 3rd GPU on its own power supply (experimenting if maybe the PSU had an issue)
Put the MB on its own Power Supply (experimenting if maybe the PSU had an issue)
Verified the 4 pin molex connectors are plugged in (the only possible issue I can think of here is I am using a splitter so one power line splits to both vs one to each)

Any thoughts would be appreciated