The Apache CXF has done some initial work to have OpenAPI v3 JSON reported from the JAX-RS endpoints.
Andriy Redko has started with the OpenApiFeature implementation which depends on the latest OpenApi v3 aware swagger-core and swagger-jaxrs libraries and demoed it here.
In meantime I did a Swagger2 to OpenApi v3 JSON conversion filter which reacts to openapi.json queries by converting Swagger2 swagger.json produced by Swagger2Feature to openapi.json with the idea of making it easier for the existing upstream code (which has already integrated Swagger2Feature) to start experimenting with OpenAPI v3, before doing the switch to the new feature (and dependencies).
This effort is still a work in progress but the results in both cases are promising. The new feature and the conversion filter will require some more improvements but you can start experimenting with them right now. And if you are someone like me then you will be positively surprised that SwaggerUI 3.6.1 and newer can handle both Swagger2 and OpenAPI v3 JSON :-).