Albert Barnes 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. Who these were is not certainly known. They may have been members of the same church with Gaius, who, for some reason, had visited the writer of this epistle. Another possibility is that they were the "brethren" who had gone from him with a letter of commendation to the church (1 John 1:9). These brethren had been rejected by the church through the influence of Diotrephes and, after being hospitably entertained by Gaius, had returned to the writer of this epistle. In that case, they would of course bear honourable testimony to the kindness they had received from Gaius and to his Christian character.

And testified of the truth that is in thee. This means you adhere steadfastly to the truth, even though errors abound and many false teachers are in the world.

Even as thou walkest in the truth. This means you live in accordance with the truth. The writer had made the same remark about the children of Cyria, to whom the second epistle was directed (see 2 John 1:4).