schema.json

{
    "$schema": "http://json-schema.org/draft-07/schema#",
    "additionalProperties": false,
    "definitions": {
        "Foo": {
            "anyOf": [
                {
                    "enum": [
                        "a",
                        "b",
                        "c",
                        false,
                        true
                    ]
                },
                {
                    "type": "number"
                }
            ]
        }
    },
    "properties": {
        "varBoolean": {
            "$ref": "#/definitions/Foo",
            "default": false
        },
        "varInteger": {
            "$ref": "#/definitions/Foo",
            "default": 123
        },
        "varString": {
            "$ref": "#/definitions/Foo",
            "default": "123"
        }
    },
    "required": [
        "varBoolean",
        "varInteger",
        "varString"
    ],
    "type": "object"
}