Do you intend on blending the data-sets or are you trying to make a traditional SQL-type join?
Also, are both data-type's the same? Unlike SQL which would imlicitly convert data-types, Tableau needs both data-types to be explicitly the same.
Thanks for the response. I'm attempting to make a traditional SQL join. All 3 data sets are from the same source and are .txt files in a CSV-like format.
Let me know if you need anything else.