First pc build - system sends signal with 2 gpus but not 3

Nov 12, 2018
Hey guys, first post here! I'm setting up my first build and I have this issue:
The pc has 2 zotac minis (pcie 1 &4) and a sapphire radeon (pcie 2).
They're not connected with sli or crossfire, the zotacs are for deep learning so looks like they should work as they are.
Sapphire doesn't need/have cable power while the other two are connected with a single cable to my aerocool psu (850W).
Only sapphire is connected to monitor via dvi.
The system can't send signal to the monitor when all 3 are on the mobo but I managed to view the bios screen when I removed the gpu (zotac) on pcie 1.
Putting it back in, no signal.

Is it because the system checks pcie 1 first before moving on to the next?
I think I've connected that gpu to the monitor before and there was no signal either but I'll double check.
Btw the cpu is a threadripper with 64 lanes so that's most likely not an issue (all pcie lanes combined are 40 plus an m.2)
Nov 12, 2018

Mobo is asrock taichi x399
Cpu threadripper 1900x
2 zotac mini gtx 1070
Sapphire radeon rx560
Ram ballistix sport 32 gb
Psu aerocool p7 850W
Monitor LG 22MP68VQ
dvi cable, also tested an hdmi
An m.2 samsung, a crucial ssd and an hdd (probably not important)
Some fans including a corsair watercooler

The zotacs are in the 16 lanes and the sapphire in 8

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