“three sons of Zeruiah were there.” Zeruiah was David’s sister (1 Chron. 2:13-16), so Joab, Abishai, and Asahel were David’s nephews.
“as swift of foot as one of the gazelles.” Asahel was a fast, smooth runner who could run long distances. People like that were valuable in the ancient world, which had no telephone or other quick method of long-distance communication. They often became royal runners.