Is the bearing capacity of the building ground sufficient at 3 meters?

It is in your own interest to have a soil survey done for your property. Only this is meaningful.

In our new development area, measurements were only taken in the area of the later street. One measurement point was located quite precisely in front of our property. However, the measurements on our property itself showed a completely different result.

Can one say how deep the soil must generally be load-bearing when using a foundation slab?

I don't think you can say that so generally. Wait for the [Bodengutachten]. It also contains a foundation recommendation. We also built only on a slab and still had to completely replace the soil up to a depth of 1m.

May I ask how much more that cost you approximately?

We only have a slab foundation and had to remove 80cm and rebuild because the required load-bearing capacity was not given. This cost us about €9,000 extra including disposal of the "bad" soil. We only kept the topsoil / sod.

We had the soil survey done before purchasing the plot and planned the additional costs accordingly.

May I ask roughly how much more it cost you?

Including removal of the excess soil approx. €10,000.