From the Encyclopedia Britannica:

1 John

The First Letter of John was apparently addressed to a group of churches where “false prophets", denounced as antichrist, denied the incarnation of Jesus and caused a secession so substantial that the orthodox remnant was sadly depleted. The faithful were deeply disturbed that the heresy found favour among pagans, and they apparently felt inferior because those who had left their midst claimed to have profound mystical experiences. The heretics asserted that they possessed perfection, were “born of God”, and were without sin. By placing themselves above the Commandments, they in fact sanctioned moral laxity. John’s letter thus urges the Christian community to hold fast to what they had been taught and to repudiate heretical teachings. Christians are exhorted to persevere in leading a moral life, which meant imitating Christ by keeping the Commandments, especially that of loving one another. The spirit of the letter closely parallels that of the Gospel According to John.

2 John

The second and third letters are closely akin to the first in language and ideas. The Second Letter of John exhorts a church, fancifully called “the elect lady and her children”, to boycott the docetic heretics (combated in 1 John), who deny the reality of the Incarnation. In 2 John, as in the Gospel According to John and 1 John, the light - darkness images are similar to those of the Dead Sea Scrolls. To “walk in the truth” is to reject heresy and follow the doctrine of Christ.

3 John

The Third Letter of John is addressed to a certain Gaius and complains that “Diotrephes, who lies to put himself first, does not acknowledge my authority” - a hint that gnostic teachings were severely disrupting the community. The writer is concerned about and has responsibility as presbyter for the missionaries of the church.