Charles Ellicott Commentary 2 Chronicles 11:17

"So they strengthened the kingdom of Judah, and made Rehoboam the son of Solomon strong, three years; for they walked three years in the way of David and Solomon." — 2 Chronicles 11:17 (ASV)

So they strengthened ... three years — that is, during the first three years of the reign. “There is no ground for suspecting the antiquity of this record. On the contrary, it was likely beforehand that the pressure from the north caused a proportionally greater earnestness in the religious life of the southern kingdom, and that the former was weakened and the latter strengthened by the migration” (Bertheau). This will also explain Jeroboam’s refraining from attacking his rival’s territory. (See Note on 2 Chronicles 11:10.)

For three years. — Literally, for they walked ... for three years. The reason for the strengthening.

They (that is, the people of Judah) walked in the way of David and Solomon — that is, they served Jehovah according to the system of worship enacted by those monarchs. The support which Solomon in his old age gave to foreign religions is here again ignored by the chronicler.

What happened after the three years of faithfulness is told in 2 Chronicles 12:0. The account of these events follows.