John Gill Commentary 3 John 1:3

"For I rejoiced greatly, when brethren came and bare witness unto thy truth, even as thou walkest in truth." — 3 John 1:3 (ASV)

For I rejoiced greatly when the brethren came
From the place where Gaius lived, to that where John now resided; these brethren seem to be preachers of the word, who travelled from place to place to spread the Gospel:

and testified of the truth that is in you ;
either of Christ, who was formed in him; or of the Gospel, which had a place in his heart; or of the truth of grace that was in him, as well as of that faithfulness, integrity, and sincerity he appeared to be possessed of, being an Israelite indeed, and without guile:

even as you walk in the truth :
in Christ, and in the Gospel, and as became it, and with all uprightness; see (2 John 1:4) ; and this occasioned great joy in the apostle; as it is matter of joy to every one that truly loves Christ, and his Gospel; or has the true grace of charity in him, which envies not the gifts and graces of others, but rejoices in the truth, wherever it is found; and especially to the faithful ministers of the word, when they hear of the truth of grace in any souls, and that such continue walking in the truth of the Gospel, and particularly those who have been wrought upon under their ministry, as follows.