1. First, create a line graph with the two lines you want to fill the area between. Select the data range for both lines and insert a line graph.
2. Right-click on one of the lines and select "Format Data Series" from the drop-down menu.
3. In the "Format Data Series" window, select the "Fill" option and choose a color for the line.
4. Repeat step 3 for the other line, choosing a different color.
5. Now, to fill the area between the two lines, right-click on one of the lines and select "Add Trendline" from the drop-down menu.
6. In the "Add Trendline" window, select the "Linear" option and check the box next to "Display Equation on chart".
7. Click "Close" to close the window. You should now see a straight line on your graph.
8. Right-click on the straight line and select "Format Trendline" from the drop-down menu.
9. In the "Format Trendline" window, select the "Fill" option and choose a color for the area between the two lines.
10. Click "Close" to close the window. You should now see the area between the two lines filled with color.
11. To add another line in between the two lines, simply add another data series to your graph and format it as a line. You can then adjust the position of the line by changing the data values.

By following these steps, you should be able to fill the area between two lines in an Excel graph and add an extra line in between.