While Craft 3 is in beta, you'll need to install plugins with Composer. If you've installed Craft 3, you should already know a bit about Composer.
To install CP Nav, follow the below instructions.
Open your terminal and go to your Craft project:
Then tell Composer to load the plugin:
composer require verbb/cp-nav
In the Control Panel, go to Settings → Plugins and click the “Install” button for CP Nav.
To edit your navigation items, go to Settings → Control Panel Nav.
Available for Craft 2 & Craft 3, unlimited use, free forever.