I have an alternate suggestion. I would create the fourth table that Mike talks about but I would set the key field to be a prefix corresponding to each of your three separate tables and then a dash and then the record ID of the source table.
In each of the three tables I would create a formula to make that key field of the fourth table so for example it would read like
List("-", "Business Unit 1", ToText([Record ID#]))
Then create three saved table to table copies to copy the fields from respectively each of the source tables into the combine table.
Then I would set a pipeline to run every hour say which would be to first delete all the records in the combined table and then to successively run each of the saved table to table copies.
The make Request step to run the saved T2T copy would look like.
mycompany.quickbase.com/xxxxxxxx/?act=API_RunImport&ID=10