How do I make Panel always transparent like the Dock in GNOME 3?

You may use a GNOME shell extension called Dynamic Panel Transparency to achieve this.

1. Install the extension from the link above.
2. Reload the page. A settings icon should appear.
3. Click on the settings icon. A window should pop up.
4. Go to the "Background" tab and "Enable custom opacity".
5. Set low values for both "Maximized Opacity" and "Unmaximized Opacity".
   

Note: You may have to restart the shell/session to see the change. In an Xorg session press alt+f2, then type r and press enter. In a default Wayland session log out and log in again.

_{Tested in an Xorg session, but not in a Wayland session.}