I am running 3 1080tis on my Evga 1000w ps with no problems. With the kill-a-Watt meter and 100% power to the GPUs it reads ~700w at the wall. 70% power and GPU men set to +266 it reads ~665w at the wall.
Power draw jumps 25-50w if I do other activities on the computer while I mine. Seems to be plenty of head room to use 3 1080ti GPUs and 1000w PS.