There are two options I see possible here:
1. Modify the file2 and file3 to have the same column headers of file1(I would highly recommend this). Then the data can be added to the extract utilizing the method listed in the Product Help here: Adding Data to Extracts
2. I was able to dig up another community post very similar to this use case. In the comments of the post they state the use of a custom join within the excel files to create a single data source.
I hope this was able to help in regards to the issue.