Custom function to create an invoice from a list of subforms

Custom function to create an invoice from a list of subforms

I want to create an invoice based on the list of subforms created in a custom module. Please tell me the custom function to be assembled.
The custom module JSON is below.
  2. {
  3.   "Owner": {
  4.     "name": "EX NAME",
  5.     "id": "3872746000000200013"
  6.   },
  7.   "$currency_symbol": "¥",
  8.   "field19": null,
  9.   "field17": "5年リース",
  10.   "field18": "見積発行より2週間",
  11.   "Last_Activity_Time": "2019-03-23T22:04:14+09:00",
  12.   "Name": "1001",
  13.   "field22": null,
  14.   "field23": null,
  15.   "field20": null,
  16.   "field21": "ご相談の上",
  17.   "field26": "貴社指定場所",
  18.   "$process_flow": false,
  19.   "field27": "別途契約",
  20.   "field24": null,
  21.   "Currency": "JPY",
  22.   "field25": null,
  23.   "id": "1234546000000239389",
  24.   "$approved": true,
  25.   "$approval": {
  26.     "delegate": false,
  27.     "approve": false,
  28.     "reject": false,
  29.     "resubmit": false
  30.   },
  31.   "Modified_Time": "2019-03-23T22:04:14+09:00",
  32.   "Created_Time": "2019-03-21T07:56:55+09:00",
  33.   "$editable": true,
  34.   "field28": null,
  35.   "field29": null,
  36.   "field11": "2019-03-21",
  37.   "field12": "リース",
  38.   "field1": {
  39.     "name": "株式会社サンプルカンパニー PC一式",
  40.     "id": "3872746000000227195"
  41.   },
  42.   "field10": 48800,
  43.   "field": {
  44.     "name": "株式会社サンプルカンパニー",
  45.     "id": "3872746000000216241"
  46.   },
  47.   "field15": "納品月末締め翌月末お振込み",
  48.   "field16": null,
  49.   "field13": "下記の通り",
  50.   "field14": null,
  51.   "field7": 62.69,
  52.   "field6": 1680000,
  53.   "field9": 1.82,
  54.   "Tag": [],
  55.   "field8": "60",
  56.   "field3": [
  57.     {
  58.       "$approval": {
  59.         "delegate": false,
  60.         "approve": false,
  61.         "reject": false,
  62.         "resubmit": false
  63.       },
  64.       "$currency_symbol": "¥",
  65.       "Parent_Id": {
  66.         "name": "1001",
  67.         "id": "1234546000000239389"
  68.       },
  69.       "$editable": true,
  70.       "field1": 200000,
  71.       "field": {
  72.         "name": "PC # Example Laptop PC",
  73.         "id": "3872746000000227048"
  74.       },
  75.       "$process_flow": false,
  76.       "field7": 1000000,
  77.       "field6": 500000,
  78.       "id": "3872746000000239391",
  79.       "field3": 100000,
  80.       "field2": 5,
  81.       "$approved": true,
  82.       "field5": 500000,
  83.       "field4": 50
  84.     },
  85.     {
  86.       "$approval": {
  87.         "delegate": false,
  88.         "approve": false,
  89.         "reject": false,
  90.         "resubmit": false
  91.       },
  92.       "$currency_symbol": "¥",
  93.       "Parent_Id": {
  94.         "name": "1001",
  95.         "id": "1234546000000239389"
  96.       },
  97.       "$editable": true,
  98.       "field1": 300000,
  99.       "field": {
  100.         "name": "PC # Example Laptop PC",
  101.         "id": "3872746000000227048"
  102.       },
  103.       "$process_flow": false,
  104.       "field7": 1500000,
  105.       "field6": 1000000,
  106.       "id": "3872746000000239393",
  107.       "field3": 100000,
  108.       "field2": 5,
  109.       "$approved": true,
  110.       "field5": 500000,
  111.       "field4": 66.67
  112.     },
  113.     {
  114.       "$approval": {
  115.         "delegate": false,
  116.         "approve": false,
  117.         "reject": false,
  118.         "resubmit": false
  119.       },
  120.       "$currency_symbol": "¥",
  121.       "Parent_Id": {
  122.         "name": "1001",
  123.         "id": "1234546000000239389"
  124.       },
  125.       "$editable": true,
  126.       "field1": 15000,
  127.       "field": {
  128.         "name": "作業費 # PCデータ移行作業",
  129.         "id": "3872746000000241031"
  130.       },
  131.       "$process_flow": false,
  132.       "field7": 90000,
  133.       "field6": 90000,
  134.       "id": "3872746000000241037",
  135.       "field3": null,
  136.       "field2": 6,
  137.       "$approved": true,
  138.       "field5": 0,
  139.       "field4": 100
  140.     },
  141.     {
  142.       "$approval": {
  143.         "delegate": false,
  144.         "approve": false,
  145.         "reject": false,
  146.         "resubmit": false
  147.       },
  148.       "$currency_symbol": "¥",
  149.       "Parent_Id": {
  150.         "name": "1001",
  151.         "id": "1234546000000239389"
  152.       },
  153.       "$editable": true,
  154.       "field1": 15000,
  155.       "field": {
  156.         "name": "作業費 # PCデータ移行作業",
  157.         "id": "3872746000000241031"
  158.       },
  159.       "$process_flow": false,
  160.       "field7": 90000,
  161.       "field6": 90000,
  162.       "id": "3872746000000241057",
  163.       "field3": null,
  164.       "field2": 6,
  165.       "$approved": true,
  166.       "field5": 0,
  167.       "field4": 100
  168.     }
  169.   ],
  170.   "field2": "PC6台",
  171.   "field5": 2680000,
  172.   "field4": 1000000
  173. }
It is some of these values that I want to pass to the invoice,
How do I pass a list of subforms ("field3") to an invoice?
Please give me some advice.