A, B, and C conduct table tennis training in the form of a group arena. Two players compete in each game, and the other is the referee. At the end of the half-day training, it was found that A had played 15 rounds, B had played 21 rounds, and C had served as referee for 5 rounds. So who was the referee for the third round in the whole training?
甲、乙、丙三人用擂台赛形式进行兵乓球训练，每局2人进行比赛，另1人当裁判，每一局的输方去当下一局的裁判，而由原来的裁判向胜者挑战，半天训练结束时，发现甲共打了15局，乙共打了21局，而丙共当裁判5局，那么整个训练中的第3局当裁判的是？
<result>3</result>