Creating a 3 Axis Line Chart

Here's something to get you going.  Still needs coloring, smoothing etc.

The key is the "resolve"  instruction at the bottom.

{
  "data": {
    "name": "dataset"
  },
  "encoding": {
        "x": {
          "field": "PERIOD_YEAR",
          "type": "nominal"
        }
  },
  "layer": [
    {
      "mark": {
        "type": "line"
      },
      "encoding": {
        "y": {
          "field": "QTY ",
          "type": "quantitative"
        }
      }
    },
    {
      "mark": {
        "type": "line"
      },
      "encoding": {
        "y": {
          "field": "GM % ",
          "type": "quantitative"
        }
      }
    },
    {
      "mark": {
        "type": "line"
      },
      "encoding": {
        "y": {
          "field": "GM $/unit ",
          "type": "quantitative",
          "axis": {"offset":50}
        }
      }
    }
  ],
  "resolve": {"scale": {"y": "independent"}}
}