You might have to create some dummy data so we can see the format of the data.
From what you've described, I'd create a measure using INDEX() and a table calculation to only show the last 3. You might also want a parameter so the user can change 3 to be any number (and set some limits on that as well).
But supplying some dummy data to show the problem easier would be a great start
first welcome to tableau and the forum
a little difficult to visualize the issue without seeing the book but see the superstore analogy attached
this will filter the data to the last 3 dates per subcategory
you can then use it to create a text chart
or a bar and whisker (or other ) chart
to your second issue the way users add data to Tableau is either through uploading a table to join (or union) with the existing data or through parameters - parameters are singled value selections from a prior set limited set - not appropriate for what you are suggesting
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rank of date.twbx 2.4 MB