Slitherine

Im at Kiev nov. 1943. Whereas until now NONE of my fighters have been attacked by AI fighters throughout 1935-oct 1943, now suddenly two 3 star Yak-3 fighters attack (and destroy) my 4,5 star A2 Fw190 in allout kamikaze fashion.
I have encountered more 3 star AI fighters, but what parameter/condition makes the AI now suddenly attack in similar condition whereas previously it avoids attacking?

The AI predicts how much damage it can cause before attacking. Before 43, most of the Soviet fighters are not very good but they start getting decent ones in 43. Also, experience makes quite a big difference to fighters, so better fighters and 3* units means they can do more damage, so the AI is more willing to attack.

Bear in mind that the AI only takes account of what it can see, which is why fighter traps and AA traps work. The AI decides to attack based on an undefended unit and then has a nasty surprise when the escort fighter or AA joins in.

Yes but... I think most of 1943 I have already encountered 3 star Russian Fighters, Yak3 as strongest. Not a single time these attacked my fighters - now suddenly they do. So it appears like some other factor plays a part...

What settings are you playing with? Normal, dice chess or chess? It can make a difference. If you were playing with the normal combat setting, and, you have a really bad roll after the first attack, then the AI will often send in additional units to try to finish you off. This applies to all types of units BTW. If you end up with a 2 or 3 strength unit exposed during the AI's turn you're probably going to lose it.