The code snippet option works for any website, including WordPress.
Two major points though.
First, you will need to add it to the HTML header portion of your site template. A child theme or header code snippet plugin would allow inserting the 3CX Live Chat across all pages and prevent your changes from being overwritten during updates.
Second, for Hosted WordPress at WordPress com my understanding is you will need at least the Business plan to perform the necessary changes. With the Business plan of WordPress Hosting, you could use the 3CX plugin and avoid modifying the HTML header.