Theophylact of Ohrid

The Evangelist aptly inserted here the account concerning Herod. He speaks as if to say: although the people thought very highly of John, Herod, "being reproved by him," to all his evil deeds "added this also, that he shut him up in prison." He narrates as if with deep sorrow and strong compassion that Herod dealt so unjustly with John, while the people held him in high regard.

Commentary on Luke ↗

Bede

But John was not imprisoned in those days. According to St. John's Gospel it was not till after some miracles had been performed by our Lord, and after His baptism had been noised abroad; but according to Luke he had been seized beforehand by the redoubled malice of Herod, who, when he saw so many flock to the preaching of John, and the soldiers believing, the publicans repenting, and whole multitudes receiving baptism, on the contrary not only despised John, but having put him in prison, slew him.

Catena Aurea by Aquinas ↗

Bonaventure

As regards the rebuke of the transgressing ruler, it is added: But Herod the tetrarch, about whom it was treated above in the same chapter, when he was rebuked by him concerning Herodias, the wife of his brother: Matthew fourteen: "John said to Herod: It is not lawful for you to have her." And concerning all the evils which Herod had done. …

Commentary on Luke, Chapter 3 ↗