Stay away from the tail gunner.
Come in on them fast.
dont fly strait at them, move around a little.
If its a Lanc shoot at them from underneath, no gunner down there.
Aim for the wings and engines, they are the weakest part of a bomber.
Just cause a gunner aint shooting at you right away doesnt mean he dont know youre there, he may be waiting for you to get closer.
when you can, shoot for the pilot of the lead plane, if you kill the pilot, that takes out all three of them at once.
The more you practice the less they will get you, but some times they will and you will know what you did wrong.