Hello Dave,
Thank you for your interest in the W3 Total Cache Pro license.
W3 Total Cache Pro licenses are valid on a per-install basis, meaning you need one for every WordPress instance you run. If you have three separate websites, you will need three licenses in total—one for each site. If you have five sites in a WordPress multisite instance, you need just one license for all five sites.
As for the dev environment, just add define('W3TC_PRO_DEV_MODE', true);
to your wp-config.php file on your non-production environments and re-save the license.
Thank you!