What do these traffic signs indicate?

Your answer is correct. The sign with two cars—one red and one black—inside a red circle means "No overtaking." The additional sign below with "3 km" specifies that this no-overtaking rule applies for the next 3 kilometers.

In German traffic rules, this means you are not allowed to overtake any vehicles for the distance indicated. This is important for safety when road conditions do not permit safe overtaking. Always obey these signs to avoid accidents and penalties.