Jamieson, Fausset and Brown on 3 John 1:9

9. I wrote--The oldest manuscripts add "something": a communication, probably, on the subject of receiving the brethren with brotherly love (3Jo 8, 10). That Epistle was not designed by the Spirit for the universal Church, or else it would have been preserved.

unto the church--of which Gaius is a member.

loveth ... pre-eminence--through ambition. Evidently occupying a high place in the Church where Gaius was (3Jo 10).

among them--over the members of the Church.

receiveth us not--virtually, namely, by not receiving with love the brethren whom we recommended to be received (3Jo 8, 10; compare Mt 10:40).