API Connect now supports OpenAPI v3. Now you can publish an API designed using OpenAPI 3.0 specification and manage its lifecycle in a similar fashion to that of OpenAPI v2.
In addition, API consumers can discover, view and subscribe to the OpenAPI v3 definition on the developer portal. Also, API developers can enforce policies and protect their API on the gateway.
The video below walks you through simple steps of publishing an OAI 3 API definition, viewing API documentation on the portal and testing the endpoint using API Connect Test and Monitor.
You can find the artifacts used in the demo here
You can read more on this on the knowledge center