Spiritualism

The third son was Arjuna. He and Karna were sworn enemies. While Karna knew his own identity as the eldest among Kunti’s sons, the rest including Arjuna were totally unaware of this.

In fact Karna promised to Kunti that she was and will ever be a mother of five sons. While the four would be Yudhishtir, Bhima, Nakula and Sahadeva the fifth could be either himself or Arjuna depending on who killed whom during the battle.