Japanese govt haven’t released much info after Kishida’s announcement, but my understanding is that border health controls remain the same but visa requirements will be relaxed from Oct 11.
Which means you’ll first need to check if your country is on the visa-waiver list when it is released (pretty sure US will be on it). Not sure though if the 3x vaccination is a visa-waiver requirement.
In terms of health controls though, you either need to get a PCR test 72-hours before departure or 3x vaccination.