Rashi: Asa's third year is counted towards Nadav's reign, and also towards Basha's reign. Radak - this is because Nadav did not complete his second year.

Radak: Our verse does not discuss the third year of Asa's reign, rather, the third year from when he began to fight Malchei Yisrael. First Asa fought Nadav for Nadav's two years, and in the third year, he fought Basha, who became king in place of Nadav. Asa and Basha had war all their days (verse 32). This war in which Basha built Ramah was in Basha's 23rd year, which was the 36th year of Asa's reign, and the 25th or 26th year of Asa's wars. He had 10 years of serenity at the beginning of his reign (Divrei ha'Yamim II, 13:23). Two of those 10 years were the last years of Yaravam. Nadav did not rule until eight years later. Or, he ruled, but his kingship was weak, and the years are not counted towards his reign. It says that he became king in the second year of Asa, i.e. of Asa's wars, which began in Asa's 11th year. All the calculations in this Sefer are from the start of his wars; those in Divrei ha'Yamim are from the start of his kingship. There it says "l'Malchus Asa", and here it says "l'Asa".

Radak citing Seder Olam: This war in which Basha built Ramah was in Asa's 16th year, one year after his war with the Kushim (Divrei ha'Yamim II, 14:8-14). There it says the 36th year, i.e. from Shlomo's death. When Shlomo married Bas Pharaoh in fourth year of his reign, it was decreed that the kingship be split for 36 years, corresponding to the 36 years that he was married to her. The kingship was destined to return after 36 years, in the 16th year of Asa's reign. He messed up, and relied on Melech Aram, and sent to him Otzros Beis Hashem. Since he did not rely on Hashem, Malchus Beis David did not return. It remained divided. ¹

'Rashi' (Divrei ha'Yamim II, 15:29, citing Seder Olam): Shlomo married Bas Pharaoh in the fourth year of his reign. ² He reigned 40 years in all. It was decreed that Aram will harass Yisrael for 36 years, corresponding to the 36 years that he was married to her, and in the end they will fall to Bnei David. This is why it says the 36th year of Asa (it was 36 years after Shlomo died). In the 16th year of Asa, after Zerach ha'Kushi fell, Melech Yisrael and Melech Aram made a Bris to fight Asa. Asa erred, and sent gold and silver to Melech Aram, and asked to make a Bris with him.

Malbim (Divrei ha'Yamim II, 15:29): The 36 years are counted from when the kingship split - 17 of Rechav'am (texts that say 'Yaravam' are mistaken), three of his son Avyah, and 16 of Asa.