Sounds like goal post shifting honestly. The mRNA clinical trials were done with 2 doses. It was sold as 90%+ effective on the 200 people they studied (erroneously claimed as over 40,000 people but I digress - that's information for only detailed oriented people to discover).
Now suddenly a 3rd is necessary? What happens when that fails, a second booster? A yearly vaccine like the flu shot (which barely works anyways)? Unfortunately for us humans, nature finds a way.
Speaking of nature, you know what confers full immunity? Having the infection and then recovering or having cross reactive immunity from a previous coronavirus strain.
Natural immunity is the undisputed champion of immunity.