John Gill Commentary 1 Chronicles 25:3

"Of Jeduthun; the sons of Jeduthun: Gedaliah, and Zeri, and Jeshaiah, Hashabiah, and Mattithiah, six, under the hands of their father Jeduthun with the harp, who prophesied in giving thanks and praising Jehovah." — 1 Chronicles 25:3 (ASV)

Of Jeduthun
Or Ethan, the sons of Jeduthun:

Gedaliah and Zeri ;
called Izri, (1 Chronicles 25:11) ,

and Jeshaiah, Hashabiah, and Mattithiah, six ;
whereas five only are mentioned; it may be thought that Shimei, (1 Chronicles 25:17) is the sixth, he not being mentioned elsewhere; it is a tradition of the Jews, that his mother was now with child of him, and it being foreseen by the Holy Spirit that he would be the chief of a course, the number six is given, as Jarchi observes; but rather, as Kimchi, he was young, and not fit to sing, yet was chosen the head of a course, until he was grown up and fit for it; to these six came up the second, fourth, eighth, twelfth, fourteenth, and tenth lots, (1 Chronicles 25:9 1 Chronicles 25:11 1 Chronicles 25:15 1 Chronicles 25:21 1 Chronicles 25:17) and these

[were] under the hands of their father Jeduthun ;
to instruct and direct them, and appoint their service to them:

who prophesied with a harp ;
or sung a prophetic psalm or hymn on that:

to give thanks, and to praise the Lord ;
for what he had done and promised, and foretold he would do for his people.