Many years ago at LHR.
Q : "Dad ... what?'s the difference between a Trident Two and a Trident Three?"
A: "The number of engines...."
Er... but that
is the difference between the Trident 2 and Trident 3. The Trident 3, believe it or not, had four engines - the original three, plus a fourth "booster" engine in the tail: a small RB162 turbojet, intended to help overcome the "Gripper's" infamous reluctance to part from the ground on take-off.