Hey Nenad,
As far as I know, the native way to do this that you can create three summary reports.
The first summary report will only show the Customer names and second report will show the categories then, the third one will show the tasks.
Now, you need to set the second summary report as a drill down report in the first one. After that set the third summary report as the drill down report in the second one.
Through this way, it will work like:
Customer 1
Customer 2
-----------------------
Customer 1
>Category 1
>Category 2
Customer 2
>Category 1
>Category 2
------------------------
Customer 1
>Category 1
*Task1
*Task2
>Category 2
*Task1
*Task2
Customer 2
>Category 1
*Task1
*Task2
>Category 2
*Task1
*Task2
Please let me know if this solution works for you.
Thanks,
Gaurav
