If you’d like to run a Python 3 shell within Emacs you could issue the following command:
It’ll then prompt you to confirm which Python version you’d want to run. You can just type 3 and click Enter again. If you use it a lot, however, it’s not going to be the most convenient of the solutions. You could automate the whole process by adding the following 2 lines to your ~/.emacs file:
(global-set-key [f5] 'run-python3)
The second line assigns a keybinding to the command. In the example above it is the F5 key.