John Gill Commentary 1 Chronicles 3:9

"All these were the sons of David, besides the sons of the concubines; and Tamar was their sister." — 1 Chronicles 3:9 (ASV)

These were all the sons of David
By his wives:

beside the sons of the concubines ;
who are not reckoned, and how many they were is not known; he had ten concubines at least, (2 Samuel 15:16) and (2 Samuel 20:3) .

And Tamar their sister ;
not the sister of the sons of the concubines, but of his other sons, and only of Absalom by the mother's side, of whom see (2 Samuel 13:1–39) .