If you were setting up an application with your own credentials, you would go into your account and pull out your "api username, password, and signature."
If it's asking you for a 3rd Party User, that sounds like it's wanting you to put in the value for the API username of the developer of the plugin. You would "grant permissions" for that username to make API calls on your account's behalf.
Which plugin are you using to setup PayPal in WooCommerce? If you use our plugin instead, the setup should be a little easier for you, and I could offer more direct help.
Angell EYE -
www.angelleye.comPayPal Partner and Certified Developer - Kudos are Greatly Appreciated!
