9 The author has already described Gaius as “dear friend” or “beloved” (τῷ ἀγαπητῷ, tō agapētō) in v. 1; he will address Gaius in the same way in vv. 5 and 11 (Ἀγαπητέ, Agapēte). This is a term of endearment and personal warmth, much as it is when used by the author as an address in 1 John 2:7.
10 πιστός 1.b offers the translation “act loyally” for this context, a usage which is not common but does fit well here. Since the author is going to ask Gaius for additional help for these missionaries in the following verse, he begins here by commending Gaius for all that he has already done in this regard. When the author tells Gaius “you demonstrate faithfulness by whatever you do” he is commending him for his faithful service to the traveling missionaries (the brothers). Gaius has assisted them, and they have now returned with a report of this to the author (3 John 3).

In other versions

Beloved, it is a faithful thing you do in all your efforts for these brothers, strangers as they are,

Dear friend, you are faithful in what you are doing for the brothers and sisters, even though they are strangers to you.

Dear friend, you are acting faithfully in whatever you do for the brothers and sisters, especially when they are strangers.

Dear friend, you are being faithful to God when you care for the traveling teachers who pass through, even though they are strangers to you.

Dear friend, you act faithfully in whatever you do for the brothers, even though they are strangers.

Beloved, thou doest faithfully whatsoever thou doest to the brethren, and to strangers;

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