Your explanation isn't specific enough to describe what actually your problem is. Does it happen with one card only plugged? As you describe in this sentence:
Now i have attached 1 of the sapphire nitro rx 580 graphic cards to the motherboard as well as to the power supply......
Or you have plugged one card while you are doing Windows installation? And everything goes smoothly at that time.
You should explain more about this: (related to the previous explanation) I felt a bit confused about that.
..but if i connect the usb riser to the motherboard the system wont turn on....!
Let's try to giving you some suggestion.Have you checked that you been plugging power from PSU to your riser?
When you were turning on your rig, are you using the card as the display? Or iGPU?
Change PCIe setting to Gen1 or Gen2 on bios. Don't forget to add extra power (the 2 Molex power to your mobo - near PCIe socket), Yes, that only required when we are using more than three cards, but in some case, that thing causing our rig won't boot (I forgot which thread that discusses it). Also, you can try CMOS reset (through battery or jumper) Good Luck!