You could try unioning the three CSV files in Tableau and then extract them if needed.
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Follow belows steps to achieve solution to your problem:
1) place all your csv in a single folder
2) open tableau-> on home page, go to more files->select any of your csv
3) you will find something all your csv together in tableau
4) make union of all csv you want to, by draging them or click on union and select required files.
learn more about union in tableau:- Union Your Data
5) click on add and add a connection to DS where your table is present and do a cross data base join
learn more about cross data base join;- Integrate your data with cross-database joins in Tableau 10 | Tableau Software
6) specify your join condition.
7) extract them adn do your visualization
learn more about extract:- Extract Your Data
I hope this will solve your problem,