Russian roulette (with a light bulb)

[PrayingMantis]

It might be a simple and obvious question, but I'd still like to get some feedback from the math experts here.

Someone plays "Russian roulette" with a gun and a light bulb, i.e, the gun is pointed to the light bulb. Before starting, he puts 2 bullets into the magazine, which has place for 5. Now he starts pushing the trigger, time after time. He does so until a bullet is shot and explodes the bulb. Each time he pushes the trigger, is "a trial".

Well, The bulb explodes. He tells you that it took no more than 3 trials.

What is the probability the bulb exploded in each of the three first trials?