Apply script on bucket aggregation doc_count

I have a query like below

    { "size" : 0,
    "query" : 
    {
    "bool":{
    "filter":[{
               "range": {
                    "TXN_DATE": {
                        "gte": "20171226","lte": "20171231"
                    }
                }},
                {
               "range": {
                    "LOCAL_AMOUNT": {
                        "gte": "200000"
                    }
                }}]
        }
        }
    ,
"aggs":{
    "by_customer":{
      "terms": {
        "field": "CUSTOMER_NO",
        "size":10000

      },
         "aggs": {
                "last_trx_value": {
                    "top_hits": {
                        "sort": [
                            {
                                "TXN_DATE_TIME": {
                                    "order": "desc"
                                }
                            }
                        ],
                        "_source": {
                            "includes": ["CARD_NO"]
                        },
                        "size":1

                    }
                }
            }
    }
}
}

which gives the response as follows -

{
    "took": 41,
    "timed_out": false,
    "_shards": {
        "total": 3,
        "successful": 3,
        "failed": 0
    },
    "hits": {
        "total": 17343,
        "max_score": 0,
        "hits": []
    },
    "aggregations": {
        "by_customer": {
            "doc_count_error_upper_bound": 10,
            "sum_other_doc_count": 17331,
            "buckets": [
                {
                    "key": "80303757",
                    "doc_count": 12,
                    "last_trx_value": {
                        "hits": {
                            "total": 12,
                            "max_score": null,
                            "hits": [
                                {
                                    "_index": "xxxxx-27122017",
                                    "_type": "event",
                                    "_id": "78831830-0ac8-42c0-9975-291f94e9f834",
                                    "_score": null,
                                    "_source": {
                                        "CARD_NO": "5520961901023096"
                                    },
                                    "sort": [
                                        1514381820000
                                    ]
                                }
                            ]
                        }
                    }
                }
            ]
        }
    }
}

How can I apply script like below on doc_count and add script result to a response?

"script": "doc_count >=3 ? 1:0"