PostgreSQL 12 and PostGIS 3.0.1: ERROR: could not access file "$libdir/postgis-3": No such file or directory on Linux

Welcome to SO.

You were close. Try installing postgis in your PostgreSQL 12 like this:

sudo apt-get install postgresql-12-postgis-3

After that you'll be able to successfully run

CREATE EXTENSION postgis;

On Arch Linux you need to install postgis-old-upgrade alongside postgresql-old-upgrade.