TLDR;

sudo apt install python-is-python3

This command effectively does a symlink, but not to any specific python version. Instead it keeps pace with future update.
So you always get the latest python3 as installed by your package manager.
Original Question at StackOverflow: https://askubuntu.com/questions/1272870/how-can-i-change-the-default-python-on-my-ubuntu-20-04-to-python3-8