From SKYbrary Wiki
The N3 Indicator is a cockpit gauge which presents the rotational speed of the high pressure (high speed) engine spool in a three spool jet engine such as the Rolls Royce RB211. The speed of this spool is referred to as N3. The N3 Indicator is usually calibrated in percent RPM based on an engine manufacturer defined rotational speed that corresponds to 100%. The indicator can be a "stand alone" analogue or digital gauge or the information can be incorporated into the aircraft Engine Indicating and Crew Alerting System (EICAS) or Electronic Centralized Aircraft Monitor (ECAM) systems if installed.