3-Legged OAuth is a form of OAuth that allows RingCentral to manage the authentication of users for an app provider. RingCentral supports the Authorization Code flow for apps that can protect a client secret and Implicit Grant flow for apps that cannot (e.g. a browser-only application). You can learn more about 3-legged OAuth in the RingCentral API Reference Auth section and in the RingCentral tutorials.