minecraft-assets/data/minecraft/recipes/iron_ingot_from_nuggets.json

{
  "type": "minecraft:crafting_shaped",
  "category": "misc",
  "group": "iron_ingot",
  "key": {
    "#": {
      "item": "minecraft:iron_nugget"
    }
  },
  "pattern": [
    "###",
    "###",
    "###"
  ],
  "result": {
    "item": "minecraft:iron_ingot"
  }
}