Why was it considered bad luck to light three persons cigarettes on one match during the years 1914-17?

Answer 1

This comes from the men in fox holes and in places where they were "in the line of fire".

It takes time for a sniper to find and focus on a target. At night, the striking of a match will attract the attention of a sniper, the time it takes to light the match, the first cigarette and the second cigarette is usually, but not always, too short for them to sight on.

The added time for the third cigarette to be lit was usually enough time for the sniper to get off a shot. As it was the third man on the match that was in the cross hairs and therefore shot, it was very bad luck to be that third man.