It is addressed to Caius, or Gaius, but whether to the Christian of that name in Macedonia (Acts 19:29) or in Corinth (Romans 16:23) or in Derbe (Acts 20:4) is uncertain. It was written for the purpose of commending to Gaius a party of Christians led by Demetrius, who were strangers in the place where he lived, and who had gone thither for the purpose of preaching the gospel (verse 7).
The language of this epistle is remarkably similar to 2 John, and it is the scholarly consensus that the same man wrote both of these letters, although it has been doubted he also wrote the Gospel of John, 1 John, or Revelation.
This article uses text from Easton's Bible Dictionary, 1897.