title: pip3: command not found but python3-pip is already installed
body: <p>I can't use pip3 though python3-pip has already been installed. How to solve the problem?</p>
  
  <pre><code>sudo pip3 install virtualenv
  sudo: pip3: command not found
  
  sudo apt-get install python3-pip
  Reading package lists... Done
  Building dependency tree       
  Reading state information... Done
  python3-pip is already the newest version.
  0 upgraded, 0 newly installed, 0 to remove and 11 not upgraded.
  </code></pre>