renewable-resources/data/renewable/recipe/chainmail_chestplate.json

{
  "type": "minecraft:crafting_shaped",
  "category": "equipment",
  "pattern": [
    "N N",
    "NNN",
    "NNN"
  ],
  "key": {
    "N": "minecraft:iron_nugget"
  },
  "result": {
    "id": "minecraft:chainmail_chestplate",
    "count": 1
  }
}