John Gill Commentary 2 Samuel 3:2

"And unto David were sons born in Hebron: and his first-born was Amnon, of Ahinoam the Jezreelitess;" — 2 Samuel 3:2 (ASV)

And to David were sons born in Hebron
He was married before he came there, had wives in his state of exile, but had no children by them there, at least no sons; if any, only daughters:

and his firstborn was Amnon, of Ahinoam the Jezreelitess
who being mentioned first, and her son his firstborn, seems to have been his wife before he took Abigail; he had not much comfort of this firstborn son of his; see (2 Samuel 13:1–39) .