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.