When registering Prism Element to Prism Central, the three parameters that must be specified are the Prism Central username, the Prism Central virtual IP, and the Prism Central password. The Prism Central username is required to authenticate the user on the Prism Central side. The Prism Central virtual IP is needed to identify and connect to the Prism Central instance. Lastly, the Prism Central password is required for authentication purposes. Therefore, these three parameters are essential for the registration process.