Signal Superposition and divider?
All the resistors act all the time. 

If your signal processing really presents and open circuit (I.e. a transistor or relay ensures that), then you 50V value is being sensed with a 3k in the lower leg; the senses value will be small and hard to sense accurately. 

If the signal processor pulls to a specific voltage, the analysis is a bit more complicated, but the small value of the 3k will likely still dominate.