In that case, since the third axis is not supported directly by excel, you'll have to create it yourself.
I've not done for years and I don't have any example at hand. I can give some hints
- the third axis will be, in fact, just a vertical line drawn by you
- you format that line like an axis, formatting the color of the line and of the marker, the width of the line and the type of the marker
- you change the labels of that line to the values you want in your axis
- the series you use for the third axis is not the original series but the original series multiplied by a scale factor
These are the main ideas.
To clarify, let's suppose that the series have these ranges
Series 1: 0 to 10
Series 2: 200 to 450
Series 3: 10000 to 20000
- choose for example series 1 and 3, build the chart and send series 3 to the secondary axis.
- build a table for the series 2 auxilliary, that will have all the values of the original series divided by 50. This means that its values will have a range 4 to 9, ok for the primary axis.
- add this aux series to the chart, in the primary axis
The 3 series are now all on the chart and thanks to the scaling we did, the second series does not disrupt the first axis
You can now add the new axis with the appropriate labels to series 2 aux.
This is not difficult, but requires some work. If you are not used to manipulating the chart please post more specific doubts, the series chart types, maybe an example of your data and more details. In that case don't forget to add your excel version.
Hope it's clear.