tableau, excel workbook, three worksheets, want data sources column in union

I have a question on Tableau Desktop. I have an excel file with three tabs: Applications, Shared Drives, Sharepoint_Libraries. I would like to union all of these tables. I would like a column called Data Source with the below values. So from Applications excel file, I would like a Data Source column in the Union which says Applications. So from Shared Drives excel file, I would like a Data Source column in the Union which says Shared Drives. Lastly, from Sharepoint_Libraries excel file, I would like a Data Source column in the Union which says Sharepoint_Libraries. How can I do this in Tableau? Do I need to edit the source excel files and add the Data Source column in each file? Or, can I create a calculated field called Data Source and use a formula to populate this information?

Data Source
Applications
Shared Drives
Sharepoint_Libraries

Solution

Tableau has a "merged mismatched fields" option for this situation.

So..

open Desktop
connect to the Excel file
drag over the Applications sheet
drag over the Shared Drives sheet and position it just below Applications until you see the "union" box appear then release your mouse button
repeat for Sharepoint Libraries
in the preview window (below) you will see the three different fields.
highlight the three fields that you want to merge
right-click and choose Merge Mismatched Fields.

Then you can rename the field accordingly :)