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Hello Juergen,
To cut to the cheese: :-))
1) Indeed a Dasysyrphus, and my guess would be a female of D. venustus, although you would have to see the face to avoid confusion with D. hilaris! D. hilaris has a yellow face, D. venustus a black stripe from mouth to antennae!
2) Certainly a male of Cheilosia, and I think this is C. albipila!
It seems to have dark hairs on the eyes, tibia 3 is yellow and the antennae are orange!
3) I think this is a male of Parasyrphus annulatus! Visible is tibia 3, which is yellow at base and dark at the top!
Greetings