N3 Speed is furnished by the dedicated Alternator, which is powered through the gearbox by the HP Rotor. Two separate single Phase N3 windings supply N3 speed to each of the two channels in EEC. EEC utilizes this data to monitor speed, control the engine, and sense N3 Overspeed.
If there is a total loss of Shaft speed signals, EEC generates a "synthesized pair of shaft speed signals" (N1 and N2) for the cockpit, and uses N3 actual to "maintain transient Control." Turbine D, this is book on the TRENT 900, the others, I do not know.