Yes, there is an easier way to create such a chart. You can use Excel's built-in PivotChart feature to create a chart with three variables.

Here are the steps to create the chart:

1. Select all of your data, including the headers.
2. Go to the Insert tab and click on PivotChart.
3. In the Create PivotChart dialog box, make sure "New Worksheet" is selected and click "OK".
4. In the PivotTable Field List, drag the first variable to the "Columns" area, the second variable to the "Rows" area, and the third variable to the "Values" area.
5. By default, Excel will create a sum of the third variable. To change this, click on the drop-down arrow next to the third variable in the "Values" area and select "Value Field Settings". In the dialog box that appears, select "Average" (or another appropriate calculation) and click "OK".
6. Your PivotChart should now be created. To add a trendline, right-click on any data point in the chart and select "Add Trendline". In the "Format Trendline" dialog box, select the type of trendline you want and customize it as desired.

That's it! Your chart should now have all three variables plotted, with a trendline for each value of the third variable.