After a few tests, i believe that my problem is no enough power. Today i made a simple test. Gpu 1 on motherboard , Gpu 2 and Gpu 3 on pcie risers. Riser 1 connected from 6pin to sata ,Riser 2 connected with 6pin to 6pin. The pc turned on ,successfully. Without undervolt consumption was about 100w per gpu ,pc freezes about 1 - 1.30 hour mining.When undervolt vdcc and mdcc 850 and 875 about 80 with 85w per gpu , seems to be stable.That points that the problem is lack of power.

Thank you for you fast answer my friend.So what do you propose ? Disconnect 1 gpu , until the Seasonic psu arrives?

*For the power issue, i meant ,maybe cx 750 is not enough for 3 gpus ? gpuz show about 95 -101 watt chip power per gpu.

May connecting risers directly to the extra 6pin ?
i have 2  pci express cables (2 8pin and 2 6pin+2) , 8pin are on the cards and i thought that maybe connect 6pins directly to risers?

good morning guys.
I started mining 2 months ago. One of  my worker (Amd ryzen 3 1200,8gb ram,ssd 120,power supply corsair cx 750m), Gpus: RX 580 sapphire nitro+ 4gb) has very strange problem (to me).
Firstly i install 2 gpus on the sli slots and 1 out of the case with pcie riser,Radeon Wattman for some reason does not 'see' 2 of 3 gpus.Instead Claymore seems to mining normally.
I tried 1 gpu on the motherboard and 2 with pcie risers. When connecting all these , the pc does not turn on.I test  pcie riser in the same line with ssd and use for the second riser, one new sata line directly from the psu.
The pc does not turn on. I tried multiple ways and i figured out that if i connect the second riser on a second sata independed line, pc does not turn on.
Riser does not have problem ,i test it ! I tried 2 risers and a ssd in the same line ,pc turned on but the ssd burned.
Finally i installed 2 gpus on the motherboard and 1 pcie riser.I turn pcie setting to gen2,seems to see all gpus but i believe that something going on with power.
I read an article about pcie riser and that they should suck power from extra 6pin of the gpu.
I have already order another psu seasonic focus 850 platinum. Please help me to figure out this problem. My target is to install 4 gpus at this worker and  to be stable.All my gpus is flashed and tested with no gpu errors.