The two were first referred to in the Peanuts comic strip from October 1, 1963, the day after the first appearance of "5". The new boy at school tells Charlie Brown that the names of all the members of his family are numbers. Charlie Brown comments that 3 and 4 are "nice feminine names." A little over two weeks later, on October 17, 1963, the twins first appeared in the strip, introducing themselves to Lucy van Pelt.
Both 3 and 4 made only a very limited number of appearances in the Peanuts comic strip. They appeared in 1963-1964 and once in 1968.
3 and 4 appear briefly but memorably in A Charlie Brown Christmas. The twin girls, both wearing purple dresses, do not speak in the animated television special but they perform a distinctive and lively dance to the tune "Linus and Lucy" played on Schroeder's piano. They also appear in You're in Love, Charlie Brown, where they are seen making sandcastles during recess, before "Pig-Pen" walks by the sandbox and carries all the sand away with him. They also make a short cameo appearance along with 5, Frieda, and Faron in Happiness is a Warm Blanket, Charlie Brown.