I was talking about the engine being operational and generating drag, not turned off. The engine wouldn't be generating useful energy while turned off at all. Just take a moment to think about it. You are talking about operating a gas turbine as if it were a wind turbine, so the air would have to run through the numerous compressor stages, then the combustors, then the turbine stages, then through the convergent-divergent nozzle, and while it does all of that, it would be operating the gearbox and running the generator and its constant drive unit (the IDG) and oil pump and sending oil to all axial and radial load bearings, which is a process that requires a gigantic amount of energy. The passive airflow would need to do this outside of the compressor map spec and the compressor would be in the surge region due to insufficient air. I could go on and on explaining other complications such as clogging up the fuel injectors with dirt due to them being idle, or the oil and it's cooler being too cold to lubricate properly (if they even do run), or the bleed or secondary air pressures and their valves and the anti-icing system that relies on these pressures and valves, but I think you get the idea of why a gas turbine cannot be operated as a wind turbine. In the real world, the engine would be stalled and air won't even enter the intake and would instead flow around it into the path of least resistance, especially since this is a mixed flow turbofan.
The other issue is having two different engines complicates logistic and maintenance, which is a wasteful solution that the Chinese are not known to entertain.
To summarize, I don't subscribe to the third engine shutting off at specific speeds or being a dedicated electrical power plant. This is not a Toyota Prius.
It is not a given. We don't have a spec sheet saying that, and judging by the three engines and the aggressive wing sweep and finness/aspect ratios compared to other flying wings such as the B-2 and B-21, I'd say there is a very good chance this aircraft would spend more time in supersonic regimes than usual.
The real problem is the third engine cannot produce thrust at supersonic speeds while still consuming fuel. It will produce drag whether it's on or off. That means that engine is just dead weight. If we reverse the situation to to slower speeds where the other two engines are producing too much kinetic energy at the nozzle, we are still suffering from low propulsive efficiency, so no matter how you spin this, you will end up with either the low bpr engine or the higher bpr engine not being efficient at the speed in question, which is why I think it is better to optimize the aircraft's body and its engines for one single speed, or have the same adaptive cycle engine, not two different engines.
According to Tinghua University, the J-20 can do 2000km with 12 tons of fuel, which amounts to 6l/km with two engines. Taking in the assumptions I made regarding speed and drag being identical to the J-20, the J-36 would still need 18 tons of fuel to do 3000km, but the J-36 is larger and heavier and may or may not go faster than the J-20 and it has one more engine drinking fuel, so 20 tons would unlikely be enough to make that 3000km trip and back, unless the three engines the J-36 uses consume around as much fuel as the WS-10s on the J-20, which I also find unlikely.
