Excel Power Query whats the best way to combine multiple Tables across multiple workbooks

It seems like you're building to a false assumption. The error is caused when table3 doesn't exist--why doesn't it exist? Is some random other file getting mixed in? Is someone upstream giving you improperly formatted reports?

If you step the code back in the PQ editor you can find the step just before the "function" is applied which brings in the file contents. You can see file-by-file where the error is, and investigate whats going on. 

You should be able to select "skip files with errors" to drop these and clear the fault, but is that right?