Just Follow these few simple steps:

• Download the Unity file from here

• Make that download file executable using following command in your terminal: chmod +x UnitySetup-2018.2.7f1

• Install the prerequisites via

  sudo apt install libgtk2.0-0 libsoup2.4-1 libarchive13 libpng16-16 libgconf-2-4 lib32stdc++6 libcanberra-gtk-module
  

• Run the installer.

  ~/Downloads/UnitySetup-2018.2.7f1
  

• I like to place it in the /opt folder once it's installed and create a link so that I can launch it from the command line.

  cd;
  sudo mv Unity-2018.2.7f1 /opt/Unity3D;
  sudo ln -s /opt/Unity3D/Editor/Unity /usr/bin/unity3d;
  

  Once that's done, you can type unity3d to launch the Unity3D Editor from terminal.