I'd add that Kim Jong Un is the first secretary of the Workers' Party of Korea, which is the majority party and follows democratic centralism, so he does have a good deal of extra power than the other two people sharing what other countries call the head of government.

However the first thing Jong Un did when he became General Secretary of the Party was to abolish the position and replace it with a committee in which he no longer had veto power. Kim Jong Il became the "Eternal General Secretary" of the WPK mirroring what Kim Jong Il did when he abolished the role of the President, made Kim Il Sung "Eternal President" and split its functions among the three positions that you mentioned.