The three oldest sons of Jesse had gone to follow Saul to the battle. The names of his three sons who went to the battle were Eliab the firstborn, next to him Abinadab, and the third Shammah.
1 Samuel 17:13
Surrounding context
17:12 Now David was the son of that Ephrathite of Bethlehem Judah, whose name was Jesse, and who had eight sons. And the man was old, advanced in years, in the days of Saul.
17:13 The three oldest sons of Jesse had gone to follow Saul to the battle. The names of his three sons who went to the battle were Eliab the firstborn, next to him Abinadab, and the third Shammah.
17:14 David was the youngest. And the three oldest followed Saul.