What do these traffic signs indicate?

You answered correctly. This sign combination means "No overtaking for 3 km." The round sign with the two cars (one red) indicates a no-overtaking zone, and the supplementary sign below states that this restriction applies for the next 3 kilometers. It does not mean the restriction starts in 3 km, but that the restriction is in effect immediately and lasts for the specified distance. This is important because overtaking is only allowed again after this distance or at an explicit end-of-restriction sign.