John Gill Commentary 1 Chronicles 23:6

"And David divided them into courses according to the sons of Levi: Gershon, Kohath, and Merari." — 1 Chronicles 23:6 (ASV)

And David divided them into courses among the sons of Levi ,
&c.] Into three classes, according to the number of them, which follow: namely,

Gershon, Kohath, and Merari ;
see (Exodus 6:16) .