Custom Parameter to use Calculated Field with IIF Statement to Switch Measures???

I have a Paginated Report I am developing in PBI RB.

I have a custom Parameter with Values I've entered manually "A", "B", or "C".

I am trying to create a calculated field that uses "Measure A", "Measure B", or "Measure C" (measures that are in the model) depending on which parameter value you select "A", "B", or "C" when you run the report. 

I am having a difficult time finding the proper syntax for this when creating a calculated field. 

What's the secret here?