The three types of authentication services that can be used to authenticate user traffic flowing through the firewall's data plane are SAML 2.0, Kerberos, and TACACS+. SAML 2.0 is a widely used protocol for single sign-on (SSO), which allows users to authenticate once and gain access to multiple systems without re-authenticating. Kerberos is a network authentication protocol designed to provide strong authentication for client-server applications. TACACS+ is an authentication, authorization, and accounting (AAA) protocol used for network access control. These three protocols are suitable for authenticating user traffic as they are supported by many enterprise networks and firewall systems.