The apostle John wrote the letters of 1-3 John probably within the same general situation, or occasion, and shows a central concern in all
three, that of preaching, and supporting, the truth and not false doctrine.
Three imperatives (command verbs) in each of the letters seem to capture the emphasis of each one. See those HERE.
Together, the letters emphasize the need, still acute today, for discernment between truth and error and how we should support the (preaching of the) truth and how we should refuse to support error.