Properly Include Three CSS Files And Three JS Files

See wp_enqueue_script and wp_enqueue_style

https://developer.www.ads-software.com/reference/functions/wp_enqueue_script/
https://codex.www.ads-software.com/Plugin_API/Action_Reference/wp_enqueue_scripts
https://developer.www.ads-software.com/reference/functions/wp_enqueue_style/

Steve is correct, the files need to be enqueued. Furthermore, to ensure they are listed in the proper order, you need to specify which files should be loaded before the one being enqueued by using the dependencies argument. To specify dependencies this way, files need to be registered in order to have a usable “handle”. Use wp_register_style() and wp_register_script(). Once registered, files can be enqueued simply by referencing their handle in the dependencies array when enqueuing the most dependent file.

Note that the WP jQuery library is pre-registered as “jquery”. This version runs in noConflict mode, so the $ shortcut is invalid unless a “noConflict wrapper” is used. Look at the user notes at the bottom of the wp_enqueue_script() docs page for specifics.