3. Walt Whitman was an American poet, journalist, and essayist.
Parallelism is the repetition of the same grammatical structure in a list of corresponding ideas. In the original sentence the author begins Walt Whitman's career titles as nouns, and then changes the last one to an independent clause. To fix the problem, the independent clause needs to change to a noun that means "he wrote essays." An author of essays is an essayist. This is the correct revision to create appropriate parallelism.