For three authors, MLA shifts to a comma-and-“and” combo. In your sentence, write Smith, Johnson, and Lee found that productivity peaks at noon. Yes, use the Oxford comma—your English teacher will smile.
For the Works Cited, list all three names: Smith, Jane, John Johnson, and Amy Lee. Then continue with the rest of the citation. Think of it as your favorite indie band’s lineup: everyone gets credit, no substitutes.