Albert Barnes Commentary 1 Kings 22:1

"And they continued three years without war between Syria and Israel." — 1 Kings 22:1 (ASV)

Three years - These must be counted from the end of Ben-hadad's second campaign (1 Kings 20:34). They were not full years, as is evident from the next verse. The first year was likely that of Ben-hadad’s dismissal after his defeat, the second was a year of actual peace, and the third was the year in which Jehoshaphat paid his visit and the expedition to Ramoth-gilead took place.

This pause in the war between Israel and Syria was perhaps the result of a common danger. It was probably in the year following Ben-hadad’s dismissal by Ahab that the first great Assyrian expedition into this region occurred. Shalmaneser II relates that on his first invasion of southern Syria, he was met by the combined forces of Ben-hadad, Ahab, the king of Hamath, the kings of the Hittites, and others. These forces gave him battle but suffered a defeat.