According to my understanding (which comes from popularized sources, not I am not a doctor nor a biologist) antibody counts are not the main drivers of long-term immunity. Lasting immunity is given by memory T and B cells, which are able to quickly escalate the immune response in case of new infection, including producing new antibodies. So while high antibody count means you’re well protected, a low count some months after the vaccine could mean that the protection has reduced, but in almost all cases you will be protected for a much longer time. Note that low antibody count immediately after the vaccine would be different, but I don’t know if this even happens in people with an healthy immune system. Unfortunately there is no easy way to test how many memory T/B cells you have against a specific virus, without even going into how responsive they are.
So I think testing for antibodies before giving third doses would still result in giving the booster it to many more people than need it. Depending on how many doses you save, and on the costs of testing vs vaccinating, it may still be worth it. But it’s probably more practical at this time to give the booster to the people we expect have developed less memory cells, in other words the immunocompromised and maybe elderly people. For the others, I would simply wait to have more data, and ship the extra doses to poor countries.