Three on a match (also known as third on a match) is a supposed superstition among soldiers during the Crimean War to World War I. The superstition goes that if three soldiers lit their cigarettes from the same match, one of the three would be killed or that the man who was third on the match would be shot. Since then it has been considered bad luck for three people to share a light from the same match.
The belief was that when the first soldier lit his cigarette, the enemy would see the light; when the second soldier lit his cigarette from the same match, the enemy would take aim and note if the soldier was friendly or foe; when the third soldier lit his cigarette from the same match, the enemy would fire. Another explanation for this was that the first to light the match gave an enemy marksman the range to the target, the second gave the windage on the target, and the third one was shot using this information.
There was in fact no such superstition during the First World War. (The light would not be visible if the soldiers were in a trench or bunker, as they usually were when not attacking.) The superstition was alleged to have been invented about a decade later by the Swedish match tycoon Ivar Kreuger in an attempt to get people to use more matches but it appears he merely made very shrewd use of the already existing belief which may date to the Boer War. In the 1916 novel The Wonderful Year the following explanation is given: “It arises out of the Russian funeral ritual in which the three altar candles are lit by the same taper. To apply the same method of illumination to three worldly things like cigars or cigarettes is regarded as an act of impiety and hence as unlucky.”