Charles Ellicott Commentary 1 Chronicles 3:4

"six were born unto him in Hebron; and there he reigned seven years and six months. And in Jerusalem he reigned thirty and three years;" — 1 Chronicles 3:4 (ASV)

These six were born to him in Hebron. —Literally, Six were born. 2 Samuel 3:5: These were born.

And there he reigned seven years. —This notice of the time David reigned first in Hebron, the Judean capital, and then in Jerusalem over all Israel, is not found in the parallel section of Samuel; but see 2 Samuel 2:11 and 2 Samuel 5:5 for the same statements.