oh I see, so instead of lowering the freq across all boards just run less of them?
You can run it without removing the other hash board but expect some wires to be melted if you run it at 120v.
And there is no problem running s9 with only one hashboard based on what I heard.
Why not adjust the working mode to try running it with 3 hashboard? If it won't fry some wires and hashboard your lucky.
The latest s9 firmware have multi-option under working mode you can under clock the unit to run it at low power mode.
Or if you want to manually set the power like I said above you need to flash it with modded firmware like Braiins OS.