Mining rig MOBO (?) problems after adding 3+ gpu.

[Leass]

Hi guys, today I've just received my next 3 1080ti plus all accessories (risers/m.2 to pcie/add2psu/psu). What was my disappointment when I've connected everything and saw my MOBO (MSI 250m Mortar) flashing VGA led and not booting (no hdmi signal at any output from cards/mobo).
I've enabled 4G decoding, changed pcie to gen 1/2 but still nothing, with more than two cards computer won't boot. Running Win 10.
I'm powering everything with 2 evga 850w, I'be checked all cards/risers.
I just need confirmation that this mobo if f*cked or any advice what could have gone wrong ^^
If it's mobo fault please suggest any not top cash tier 1151 socket mobo which supports 5-6(with m.2) cards without any problems?
Thanks in advance !

[Undefined31415]

What other troubleshooting steps have you already taken?

If you can get it running with fewer cards, the problem is most likely not your motherboard being defective. (Granted, you could have something weird going on with the slots, but that's pretty uncommon.) When you add more cards, connect your monitor only to the card in the primary x16 slot. (This is typically the physical x16 slot closest to the CPU.)

Have you checked the balance of the load distribution between your two PSUs?

Some people who use the add2psu devices say that you should always connect a card's PCIe 6 and/or 8 pin power connections to the same PSU as the one that powers the riser associated with the card. Others say it doesn't matter, but it's worth a shot. I've never used the add2psu devices for my dual-PSU setups, so I don't have personal experience there.

[Leass]

Well, I've checked all cards and risers if they are working correctly and they do, as long as all are not connected at once. I've disabled integrated gpu, switched off all unnecessary bios devices (audio ).
I've tried without add2psu, with jumper on pins and powering psu on manually but still nope.
The cards got power from psu (Zotac &Asus ROG have leds on backplate that indicates if there is a power), they starts spinning fans and flashing they rgb lights, but mobo just freezes at black screen or no output at all.

I've read somewhere that this chipset use more pcie lanes that is occupied by expansion cards*.
I'm not crossing psu's, all risers are powered with the same one that cards are powered from.


*just read it somewhere