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.

Remember, these vaccines were developed for the original variant, goal post have not been shifted. Reality is. Nobody imagined that r0 would grow from 2.5 to 6.4 in such short time. Even infection does not give full immunity but so far it appears to be better than vaccines. Besides it comes with very heavy cost without vaccination. With vaccination it might be possible without heavy losses.