A.T. Robertson 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)

I rejoiced greatly (εχαρην λιαν). As in 2 John 1:4; Php 4:10, not epistolary aorist, but reference to his emotions at the good tidings about Gaius.

When brethren came (ερχομενων αδελφων). Genitive absolute with present middle participle of ερχομα, and so with μαρτυρουντων (bare witness, present active participle of μαρτυρεω). Present participle here denotes repetition, from time to time.

To the truth (τη αληθεια). Dative case. "As always in the Johannine writings, 'truth' covers every sphere of life, moral, intellectual, spiritual" (Brooke).

Even as thou walkest in truth (καθως συ εν αληθεια περιπατεις). "Thou" in contrast to Diotrephes (verse 9) and others like him. On περιπατεω see 1 John 1:6 and on εν αληθεια see 2 John 1:4.