Three on a match

Exactly... if you light a cigarette in the dark (trench)- then pass it to the left or right, a sniper will then know which way the match is going.. and....aim and fire...not good, in battle!

Does anyone know where the phrase "three on a match came from"?

Trench warfare. If you give a sniper enough time (say, for instance, enough time to light three cigarettes on a single match) he can focus in, take aim, and kill you. Hence, the third on the match will have seriously Bad Luck.