Is there a generator to convert OpenAPI 3.0 to Swagger 2.0?
Mashery, an API gateway, requires Swagger 2.0 format on input to open endpoint.
LucyBot api-spec-converter (online version, GitHub repo, Node.js module) can convert from OpenAPI 3.0 to 2.0.
API Transformer (paid service) also claims to be able to convert OpenAPI 3.0 back to OpenAPI 2.0. It has a command-line version too.
Keep in mind that OAS3→OAS2 convertion is lossy in general, because OAS3 has features that did not exist in OAS2 (such as multiple servers, oneOf/anyOf, different schemas per media type, objects in query string parameters, cookie parameters, and others).