One of my fellow musiscians runs Linux Mint 19.1 Tessa.
Is there a simple way to get Audacity 3 on his OS?
The best way is to build Audacity from the source code.
See: https://forum.audacityteam.org/t/building-audacity-2-4-2-on-ubuntu-20-04/57719/1
Other than that I’d recommend waiting for the version in Mint to be updated.