Jamieson, Fausset and Brown on 3 John 1:5

5. faithfully--an act becoming a faithful man.

whatsoever thou doest--a distinct Greek word from the former "doest": translate, "workest": whatsoever work, or labor of love, thou dost perform. So Mt 26:10, "She hath wrought a good work upon me."

and to strangers--The oldest manuscripts, "and that (that is, and those brethren) strangers." The fact of the brethren whom thou didst entertain being "strangers," enhances the love manifested in the act.