the heat does not turn on in the 3rd row of my 2017 Nissan Pathfinder.

Hello, The rear air conditioner is an independent system, and there is an actuator that controls the air temperature. It does this by blending the air from the heater core and evaporator to meet the demand of the passengers. If the temperature is not changing to the desired setting, you may have a bad fuse, or the blend door actuator may have failed. The good part is that you have a 2017 model, so make them fix it under warranty.