As the percentage for black folk is only three percent, if there were ten folk in an advert, how many black folk would you allow, one or none?
Perhaps there would need to be different versions of every advert? Version A would have only non-black folk and Version B would have a black person replacing one of the non-black folk for example.
That way, they could show the two versions such that when the appearances are all added together, no more than three percent of the folk in them are black.
As this would have to be done for all the other minorities, there would need to be several versions of every advert.