In what situation is it especially dangerous to perform a turn?

The combination you've selected is correct because each of the situations listed poses unique dangers when making a turn.

1. Turning before the crest of a hill can obscure your view of oncoming traffic and obstacles, making it difficult to assess the safety of the turn.
2. Turning before bends in the road similarly limits visibility, as you may not be able to see if there’s another vehicle approaching around the bend.
3. In foggy conditions, visibility is significantly reduced, which increases the risk of not seeing other road users or hazards until it is too late.

In German traffic regulations, it's crucial to be aware of such conditions as they can drastically increase the likelihood of accidents. Driving safely requires not only understanding the rules but also being able to judge when conditions may require extra caution.