You could use the List concatenation operator to combine several expression that choose a File column from each of that tables. I'll give a very generic example - you will need to fill in the details based on your data structure and implementation. Something like:
SELECT(Table1[File1], <<selection criteria>>)
+
SELECT(Table2[File2], <<selection criteria>>)
+
SELECT(Table3[File3], <<selection criteria>>)
This expression will result in a list of File column values which AppSheet will use to retrieve and attach to the email.
There are many other ways to accomplish the same thing.
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