many thread asking same question. I hope in the future you can calculate how much needs powering.

Let's play :
first, something missing here, you didn't inform us your CPU spec like (Processor, RAM, HDD/SSD)

because every part different power consumptions depend on the type, like processor Celeron VS i7.

But 1070 has TDP 150 W and you ask :
for 3 GPUS
3x150 = 450W, remaining 300 W enough for other parts powering (Proc, RAM, Riser Power, HDD/SSD).