Wish they would just make it available to all. I don’t fall into the eligible group but I’ll sure as hell take a booster.
Practically, the third Pfizer/BioNtech shot is available to all adults, since the most one may need is self attestation of eligibility. One of the criteria is working in a setting that puts you at increased risk of exposure to SARS-CoV-2, which is not a high bar, especially in areas with high prevalence. Besides, nobody is going to check these forms, and some providers may not even request them.
However, it doesn't make much sense to get a third dose before approximately six months, since the level of immunity from the initial two doses is probably still high. In that circumstance, the shot may largely be wasted, and there are still many people who haven't been vaccinated at all.
Vaccination alone won't get us out of this pandemic, though. In addition to high uptake (unlikely in the US because of the anti-vax community), we'll need common sense measures like masking and especially testing. The latter is in a sorry state, especially compared to Europe, where tests are plentiful and cheap or cost-free. Part of the problem is the way the FDA regulates COVID-19 tests as medical devices.