I tried, it does not work on the B3. Both fans still at max speed.

Tried this:

"bitmain-fan-ctrl" : true,
"bitmain-fan-pwm" : "50"

and

"bitmain-fan-ctrl" : true,
"bitmain-fan-pwm" : "50",

I thought a comma , was missing behind the "50" but no change ...