Albert Barnes on 3 John 1:11

Beloved, follow not that which is evil, but that which is good. There can be no doubt that in this exhortation the writer had Diotrephes particularly in his eye, and that he means to exhort Gaius not to imitate his example. He was a man of influence in the church, and though Gaius had shown that he was disposed to act in an independent manner, yet it was not improper to exhort him not to be influenced by the example of any one who did wrong. John wished to excite him to acts of liberal and generous hospitality.

He that doeth good is of God. He shows that he resembles God, for God continually does good. See the sentiment explained 1Jn 3:7.

He that doeth evil hath not seen God. 1Jn 1:8 3Jn 1:9. 3Jn 1:10

(*) "follow" "imitate" (b) "not" Ps 37:27 (c) "He" 1Jn 3:6-9