דברי הימים א, פרק כ״ג, פסוק ו׳

As David prepares to establish the official duties of the Levites, he brings order to the tribe. The general consensus among commentators is that he achieves this by separating them into distinct, clearly defined parts. He organizes the tribe into formal divisions structured carefully according to their specific ancestral houses [ביאור שטיינזלץ].

To complete this organization, David divides the entire tribe into three primary branches. These main divisions are based directly on the three sons of Levi, the son of Jacob: Gershon, Kohath, and Merari [מצודת דוד, ביאור שטיינזלץ].