What did David do with the water three mighty men brought him from Bethlehem's well?

Answer: Poured it out to the LORD

They broke through the Philistine camp to get it. David refused to drink, calling it "the blood of the men who risked their lives."

“So the three mighty men broke through the Philistine camp, drew water from the well near the gate of Bethlehem, and brought it back to David. But he refused to drink it; instead, he poured it out to the LORD,”— 2 Samuel 23:16 (BSB)