How to get mobo to see more than 2 GPUs

[dude77]

I am migrating a working GPU/PSU setup that I had on a windows 7/ Intel i5 computer. On my new system I can only get it to use two gpus.

The BIOS setup screen will come up one any of two GPUs I have in the motherboard but the third gives a blank screen. I have tried swapping the order of the GPUS, they are all working fine but the motherboard only sees two.

I've tried it with riser cards and mounting them directly to the motherboard. All the GPUs are powered correctly from two PSUs. I have also tried with with a 3rd PCIE video card that doesn't need power and it gives a blank BIOS screen when it's GPU number 3 but it works fine when it's the only one on the mobo.

Is there something I need to do in the BIOS such as enabling additional PCIE lanes?

I'm using:

Motherboard: GIGABYTE X299 UD4
CPU: Intel Core i7-7820X
PSUs: 2 x 750 W
GPUs: 3 x 1080ti

Thanks for any ideas you can offer

[dude77]

Gigabyte support had the answer to this:

There are not enough lanes on the CPU with the 7820x, please use the 7900x cpu if you wish to use 3 GPU.
You can relate to our website for the configurator chart

https://www.gigabyte.com/FileUpload/global/MicroSite/473/x299.html

[BCEmporium]

I'm not 100% sure about this, but I believe you can change the lanes in the bios of your motherboard. Basically, you'll want to assign less lanes per card than the typical x16 PCIE.