You can add the different shapes using a dummy filler measures and drop them to the columns shelf.
You can then format them individually to use different shapes and colours.
I have created 3 measures -SeverityValue,DetailsValue and ActivityValue and used a FillerValue with 0 values for the positioning.
Please find the updated workbook attached.
2 lists with 2 drills2.twbx 4.2 MB