Ubuntu can't find an executable file in ~/.local/bin

~/ is a shell abbreviation for your home folder, usually /home/USERNAME/, where USERNAME is the name of your user. It's the same as $HOME

~/.local/bin is a subfolder named bin in the subfolder named .local in your home folder.

You can use

~/.local/bin/pip3

to call your new pip3 executable. It may be easier to add ~/.local/bin to the $PATH environment variable, see How to add a directory to my path? so you don't need to type ~/.local/bin/ all the time.

Make sure you add it before the old value of $PATH, like

PATH="$HOME/.local/bin/:$PATH"

~/.local/bin has been added to the PATH in Ubuntu 16.10, and backported to 16.04. See https://bugs.launchpad.net/ubuntu/+source/bash/+bug/1588562

If you're running Ubuntu >=16.04 with all updates, it should already be fine. If not, you can add it manually :

For a single user, edit the file ~/.profile, and add at its end :

PATH="$HOME/.local/bin:$PATH"

If you want any newly-created user to have this, you can add this same line at the end of /etc/skel/.profile