All a Mac user has needed to do was install from https://www.python.org/downloads/ and then run python3 in the shell. Even if you use MacPorts or brew or conda or whatever, there's a distinct command to run Python 3 instead of 2.
I get that Python's package manager situation is terrible, but like the other user said, you only need built-in packages to spit out a config json or whatever.
So you would then you end up with three Python3 installations.
And if you install from the website it doesn’t override the path. So will still be using the Apple or Homebrew one.
If you install it 3 times then yeah, but even then, all 3 of them will still work.
But I could've sworn the python.org installer set the PATH. If not, that's kinda annoying.
Hence the advice to stick to the standard library - because then it doesn't matter all that much. I'm not sure what the full set of environments I've tested my Python 3 scripts actually is, but they've run OK on all the various cloud CI systems I've tried, plus my laptops, VMs, and work PCs.
Up until recently Apple only included Python2 and so developers used Homebrew to install Python3. Now it’s very common to find two versions of Python3 installed on a Mac developer’s laptop that conflicts with each other.
You really want to be using virtualenv.