Final answer:
Hydraulic system 3 pressure is cut off from the main and nose gear selector valves if system 3 pressure drops below 2100 psi to ensure safety and reliability, as the system may not have enough force to operate the landing gear correctly.
Step-by-step explanation:
The reason why hydraulic system 3 pressure is cut off from the main and nose gear selector valves if system 3 pressure drops below 2100 psi is due to safety and reliability concerns. Hydraulic systems are critical to the operation of aircraft landing gear, and maintaining adequate pressure is crucial for performing the extension and retraction functions reliably. If the pressure in the system falls below a certain threshold, in this case, 2100 psi, the system may not have enough force to overcome the weight and resistance of the landing gear, leading to potential failure. Therefore, to prevent a partial or dysfunctional operation of the landing gear, the system is designed to cut off pressure from the gear selector valves, triggering backup systems or alerting the flight crew to take appropriate action.