Please note that the CDC does not currently recommend anyone who has gotten the Johnson & Johnson vaccine to receive a second dose at this time. In addition to this, it is also important that one’s third dose of mRNA vaccine be the same vaccine as the two that they previously received. If the mRNA vaccine product given for the first two doses is not available or is unknown, then the medical professional administering the vaccine can give either mRNA COVID-19 vaccine product under this circumstance.
In closing, please realize that this is not the "booster" dose that is currently being discussed for the general population. If you have questions about whether or not you qualify, please reach out to your primary care doctor or specialist. An individual will not be able to receive the vaccine if he or she does not qualify per the guidance listed above.