I will try to explain what I have to do.

During the year, I have to report the total quantity of employees together with all branches at regular intervals. The list of employees is updated once a week. The summary list must also include lists from previous weeks.
Files A, B & C represent individual branches. The quantity of employees in individual branches is not the same. A new branch may be created during the year, in which the list of employees will start with a different week.
Files A, B & C are for demonstration. We currently have 9 branches, each with a separate file.