Albert Barnes 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)

Three years - that is, during the first three years of Rehoboam’s reign. In the fourth year, an apostasy took place which neutralized all the advantages of the immigration. In the fifth year, the apostasy was punished by the invasion and success of Shishak (2 Chronicles 12:2).