Jamieson, Fausset and Brown on 3 John 1:12

12. of all men--who have had opportunity of knowing his character.

of the truth itself--The Gospel standard of truth bears witness to him that he walks conformably to it, in acts of real love, hospitality to the brethren (in contrast to Diotrephes), &c. Compare Joh 3:21 "He that doeth truth cometh to the light, that his deeds may be made manifest that they are wrought in God."

we also--besides the testimony of "all men," and "of the truth itself."

ye know--The oldest manuscripts read, "thou knowest."