and the card (3090 suprim x) did not come with any adapter.
Well, Nvidia spec for RTX 30-series is to use 12-pin PCI-E. E.g all Nvidia FE RTX 30-series GPUs have it.
But AIBs can use different connector, like regular PCI-E connectors and it seems, that MSI, for it's Suprim X version, has done that. Your GPU has 3x 8-pin PCI-E connectors.
As how to connect the power cables, you have 2 options;
1.
As suggested by MSI (on PSU specs page)
link:
https://www.msi.com/Power-Supply/MPG-A850G-PCIE5
Meaning that you use:
* one
PCIe (16 PIN to 6+2 PIN) Cable x1
* one
PCIe (8 PIN to 6+2 PIN) Cable x2
This will leave you one 6+2-pin cable unconnected, dangling near the GPU power connectors. Up to you, which of the total 4x 6+2-pin cables you plug to the GPU. Doesn't make a difference.
16-pin and 8-pin side plugs to PSU while 6+2-pin side plugs to GPU.
2.
Or use:
* two
PCIe (8 PIN to 6+2 PIN) Cable x2
End result will be the same, one 6+2-pin cable will remain unconnected.
8-pin side plugs to PSU while 6+2-pin side plugs to GPU.
Power delivery wise, there is 0 difference since both
PCIe (16 PIN to 6+2 PIN) Cable x1
and
PCIe (8 PIN to 6+2 PIN) Cable x2
can sustain up to 288W per cable, regardless if you connect one or two 6+2-pin cables to GPU.
Oh, interesting fact about your MSI PSU - it has no warranty.
There is 0 word on PSU official specs page, on how long the PSU warranty is, or if PSU even has a warranty.
In this sense, MSI is the only one that i know of, who doesn't say anything about PSU warranty on PSU's official specs page. All other reputable PSU brands/OEMs do.
E.g Seasonic PRIME TX:
https://seasonic.com/prime-tx/
I have TX-650 (two actually). Warranty is clearly stated at the top of the page and on the bottom too, within PSU specs.