openapi: 3.0.3
info:
  title: Spec #1
  version: "0.1"
paths:
  "/test1":
    post:
      summary: POST endpoint #1
      requestBody:
        content:
          "application/json":
             schema:
               $ref: "#/components/schemas/Schema1RefTo3"
      responses:
        "200":
          description: Success
          content:
            "application/json":
              schema:
                $ref: "test2-openapi.yml#/components/schemas/Schema2"
components:
  schemas:

    Schema1:
      type: string

    Schema1RefTo3:
      $ref: "test3-openapi.yml#/components/schemas/Schema3"