b.yaml

openapi: "3.0.0"
info:
  version: 1.0.0
  title: Test
servers:
  - url: http://localhost/
paths:
  /test:
    post:
      requestBody:
        content:
          application/json:
            schema:
              $ref: "#/components/schemas/Test"
      responses:
        '200':
          description: test
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/Test"
components:
  schemas:
    Test:
      oneOf:
        - type: object
          required: ["tag", "prop_A"]
          properties:
            tag:
              enum: ["A"]
            prop_A:
              type: string
        - type: object
          required: ["tag", "prop_B"]
          properties:
            tag:
              enum: ["B"]
            prop_B:
              type: number
        - type: object
          required: ["tag", "prop_C"]
          properties:
            tag:
              enum: ["C"]
            prop_C:
              type: number