2 Samuel 23:13-17
13-17One day during harvest, the Three parted from the Thirty and joined David at the Cave of Adullam. A squad of Philistines had set up camp in the Valley of Rephaim. While David was holed up in the Cave, the Philistines had their base camp in Bethlehem. David had a sudden craving and said, "Would I ever like a drink of water from the well at the gate of Bethlehem!" So the Three penetrated the Philistine lines, drew water from the well at the gate of Bethlehem, and brought it back to David. But David wouldn't drink it; he poured it out as an offering to God, saying, "There is no way, God, that I'll drink this! This isn't mere water, it's their life-blood—they risked their very lives to bring it!" So David refused to drink it.
This is the sort of thing that the Three did.
18-19Abishai brother of Joab and son of Zeruiah was the head of the Thirty. He once got credit for killing three hundred with his spear, but he was never named in the same breath as the Three. He was the most respected of the Thirty and was their captain, but never got included among the Three.