Case Switch accepts 0,1 or 2 values for PortIndex flow variable. So you can use Rule Engine Variable node to create new flow variable with above values based on your SAP System Landscape.
Additionally, all of your 3 path are going to write the same data to an Excel, so what you are trying to do might be irrelevant, unless the only thing you are doing here is writing the same data but in different locations depending on the path. But data-wise, you are not doing anything different for each of the paths.