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I refrain from manually typing lists in drop down boxes when I can, so I have a little App_1 prep that created the drop down list. I left it in my workflow but disabled it in a container since it's not needed after dev technically.
The first dropdown in App 1 is populated from the incoming data. The second and third are output from the prior app and imported as an external source in the Dropdown tool. From there, it's simple filtering, outputting two files from the first two apps (not required in the final app) which are the data file and the file to populate the dropdown tool with the available selections.
Then simply set up your interface designer to allow the chaining and Robert's your father's brother.