Rashi: On Shabbos, a new Mishmar of Kehunah and of Leviyah entered, and the previous ones left. The entering Mishmaros will divide into three parts. One will be in Beis ha'Melech to guard lest one (Radak - of Asalyah's supporters) rise against the king, to kill him.

Radak: Yehoyada told them to guard the king. A third of the guards will be from the entering Mishmar, and two thirds from the leaving Mishmar. This is because the leaving Mishmar has more free time; the entering Mishmar needs to serve in the coming week.

Malbim: A third of the entering Mishmar will guard Beis ha'Melech from the inside, and two third from the outside (verse 6). In Divrei ha'Yamim II, 23:4,5 it explains that those two thirds guarded Sha'ar ha'Sipim and Sha'ar ha'Yesod.