1 Samuel 17:12-14

12Now David was son of a certain Ephrathite man of Beth-lehem of Judah, whose name was Jesse. He had eight sons and during the days of Saul the man was old, advanced in years among men.

13Now the three oldest sons of Jesse had already left and gone after Saul to the battle; the names of his three sons who went to the battle were Eliab the firstborn, and second to him Abinadab, and the third Shammah.

14David was the youngest. So the three oldest followed Saul.