You are actually right, I used the bottom slot for motherboard and used top right for the CPU. I've attached the correct layout now, please disregard the other image. It looks like I only have two slots left for the gpu's but none of them works after 3rd card.
https://i.imgur.com/tUtn0Qw.pngI'm also pretty sure it's not about motherboard because I can't even get to the bios screen it simply doesn't boot. (I've made the changes on the motherboard to make it support 3+ gpus)
I still have not find a solution but I made some research and it appears this PSU indeed supports multi-rail and single rail and comes multi-rail as default, however the issue is I'm using a linux distribution and not sure how can I change it to single rail without booting up windows.
My plan is the create a bootable windows Usb stick and change it to single rail but not sure if it would revert back to multi-rail once I unplug the power cable for adding additional GPU's.
If all fails I guess I'll return it and get an evga PSU.