Yeah, I figured that out too:)
2 hashboards connected = less heat and noise, and it can accept any power supply.
3 hashboards connected to APW3 power supply without 3-pin cable = hashrate is low, 8.5 GH/s from all three boards, average per board goes down drastically with same power draw as 3 fully powered hashboards with original power supply.

I don't know if this is the design, so you must purchase original power supply from WhatsMiner. I do know that my miner was faulty and I had to replace it. I had to pay 80 USD for warranty replacement (if you pay for it, it's not really warranty, is it?), and then pay them again for shipping. At least, they fixed it and send me back unit, which was hashing fine with 3 boards + original power supply. I don't have it any more so I can't test it to see, if fully working unit can have other PSU and still work fine with 3 hashboards.