וּבִשְׁנַ֣ת עֶשְׂרִ֔ים לְיָרָבְעָ֖ם מֶ֣לֶךְ יִשְׂרָאֵ֑ל מָלַ֥ךְ אָסָ֖א מֶ֥לֶךְ יְהוּדָֽה׃
And in the twentieth year of Jeroboam king of Israel began Asa to reign over Judah.
Rashi
Three years he reigned in Jerusalem and no more, as is proven in I Kings (15:9): “And in the twentieth year of Jeroboam, king of Israel, Asa reigned over Judah.” We find that he did not reign even three whole years.