I’m in 4/4
Then there will be a polyrhythm 3 over 4. So 3 beats played over normal 4 beats. So to say a half-note triplet over 4/4.
The half note under the triplet becomes the new quarter note. Which means the new tempo of the whole section which follows will be 3 over 4 where as the 3 half notes under the triplet become the new quarter notes of a 3/4 bar.
Is there any way how to tell dorico to do what I want - so that Dorico changes the tempo automatically or do I just solve it with maths and set a new tempo marker?
Thanx a lot.
No, there’s no way for Dorico to express this kind of metric modulation at present.
…but you might find this thread useful for a workaround: Swing indications using system text - Dorico - Steinberg Forums
yeah, I knew the thread - that doesn’t help for playback 
- but thanx anyway