I think that the thing to keep in mind here is that the sequential aspect of this is purely an artifact of the game system -- i.e., the "to hit" rolls.
IRL, all three gunners would be firing at the fighter more or less at the same time, so there would be no way for anyone to tell who actually scored the decisive hit.
This becomes even more complicated in that, while the game doesn't address it, there could be gunners from other bombers also firing at the same fighter.
Mark has the right idea.
But to his last point, as Andrew mentioned above, the optional rules 7.6 Awards does address multiple bombers getting credit for a kill (it's a die roll like everything else in this game).