A third dose is specifically for individuals who are moderately to severely immunocompromised and may not have built up an adequate protective immune response with their first series of vaccinations. Currently, the only authorized third doses are for the Moderna and Pfizer Comirnaty vaccines.
The recommendation is for immunocompromised individuals to receive a third dose of the same mRNA vaccine at least 28 days after receiving the second dose in the series (for individuals 18 years and older for Moderna and 12 years and older for Pfizer Comirnaty).
At this time, the CDC does not have enough data to suggest an additional dose of the Johnson & Johnson vaccine would produce an enhanced immune response in an immunocompromised person.
A booster dose is for people whose immunity may be fading after they complete their first vaccination series. There is currently no COVID-19 vaccine authorized as a booster.