If II John was addressed to the Universal Church at large, III John appears to be addressed to a specific man, named Gaius, who John commends for his character and consistent demonstration of love. This stands in stark contrast to the description of another man, Diotrephes, who actually tried to usurp authority, and who drove others out of the church. Demetrius is also commended.
Here is an outline for III John:
I. Greetings (1:1)
II. Commendation of Gaius (1:2-8)
III. Condemnation of Diotrephes (1:9-11)
IV. Commendation of Demetrius (12)
V. Concluding remarks (1:13-14)