Looks as if you're confusing the MS Jet engine which Tableau uses.
Options are either load the data into a database, or if you prefer to leave it in Excel you can do it with a custom SQL statement to UNION the three sheets together.
The easy way to do that is to connect to the workbook and initially choose one of the sheets for a single table connection - SalesOut1 say. But rather than save the connection, change it to Custom SQL. Edit the Custom SQL and it will initially have a SELECT statement which lists all of the fields from sheet SalesOut1. Copy that statement and paste it back in so you have 3 copies of the statement one after the other. Insert lines between the statements saying "UNION ALL". Edit the text of the second and third copies, replacing all references to "SalesOut1" with "SalesOut2" and "SalesOut3" respectively. that should give you what you want.
It may well be very slow - so you may need to create an extract - but at least it gets you the data you want without having to worry about creating a new database.
Thankyou Richard. I just need to get as far as an extract. I think that'll be a feat in itself rather than trying to blend Excel and Microsoft Analysis Services data (which would be the next stage), the latter of which is temperamental at the best of times :-)