You need to load the sheets separately, but you can easily concatenate the sheets on top of each other after/while loading. In order to be able to separate what data comes from what sheet you can add a '3 Months' asSheetID in your load script.
I don't know what you mean by subfields. But if you are talking about the fields that should only be in one of the tables. You should start by adding a where condition using the flag and then you will have to delete fields that have only null values.