In case that joke is a but obscure, they stand respectively for
Commander of the Order of St Michael and St George,
Knight Commander of the Order of St Michael and St George,
Knight Grand Commander of the Order of St Michael and St George.
And just to further clarify - the Order of the Garter is the premier order of chivalry in England and Wales and its memership is drawn from all walks of life and the Royal Family. Members of the Order of St Michael and St George are principally drawn from the diplomatic corps.