Final answer:
Mode 3 of the ACES II ejection system is used for low-altitude, low-speed emergencies during level flight.
Step-by-step explanation:
Mode 3 of the ACES II ejection system in an aircraft refers to the seat operation in which the pilot is ejected from the aircraft while it is in level flight.
This mode is specifically designed for low-altitude, low-speed emergencies and is used when the aircraft is flying below 200 knots indicated airspeed (KIAS) and below an altitude of 15,000 feet.
In Mode 3, the seat is propelled by a rocket motor and is intended to provide a safe escape for the pilot in critical situations.