Come ooon, 3 Stukas is a nice catch.
Tho, you gotta learn to be more patient! Judging from the pic, you opened fire from 400-600 meters? Damnit, Krat! Havent you ever heard the quote from Erich Hartmann (or was it Galland? Aaaanyways!) 'When you think you are too close, get closer' or something like that. Especially when flying with limited ammo, shooting from afar is a waste of ammunitions. I have my converge set to 120m for both MGs & cannons, I usually open fire from 150-250m, depending of the target.
A good way to practise the distances is to set up a quick mission with an friendly plane of your choice and turning the icons on.