How do I hide GNOME 3 panel?

Using the GNOME extension called Hide Top Bar.

Configure the extension in your Firefox or with Tweak Tool

I was able to achieve the behavior you want in the following way in my Ubuntu 18.04.

1. Installed Dash to Panel GNOME Shell Extension.
   To install it, simply search for "Dash to Panel" in Ubuntu Software.
2. Once installed, right clicked on "show apps" button on task bar and clicked "Dash to Panel Settings".
3. I chose Panel Intellihide "On".
4. You may edit more settings by clicking "settings button" adjacent to Intellihide

Unite Extension

try https://extensions.gnome.org/extension/1287/unite/. It brings back the Unity Shell look and feel to Ubuntu 18.04.