According to Numbers 1:3, only men older than twenty were generally permitted to go to war. This may explain why only three of David’s brothers are in the camp. Alternatively, each family sent a limited number of sons with the rest kept at home to help with various tasks and errands.
13 The three oldest sons of Jesse had followed Saul to the battle. And the names of his three sons who went to the battle were Eliab the firstborn, and next to him Abinadab, and the third Shammah.