Albert Barnes on 3 John 1:5

Beloved, thou doest faithfully. In the previous verses the writer had commended Gaius for his attachment to truth, and his general correctness in his Christian life. He now speaks more particularly of his acts of generous hospitality, and says that he had fully, in that respect, done his duty as a Christian.

Whatsoever thou doest. In all your intercourse with them, and in all your conduct towards them. The particular thing which led to this remark was his hospitality; but the testimony respecting his general conduct had been such as to justify this commendation.

To the brethren. Probably to Christians who were well known to him--perhaps referring to Christians in his own church.

And to strangers. Such as had gone to the church of which he was a member with a letter of commendation from John. Rom 12:13, and Heb 13:2.

(c) "faithfully" 1Pet 4:10