I'm afraid the [Project] field used in the grouping will not recognize any subprojects as separate from the project. The best I can suggest (other than being very careful about multiple layers of subprojects) is to manually code the sub-subproject tasks in some manner and apply the third level group to pick up those codes.

May I ask why you have some a complex structure? My concern is the possibility for corruption in the files unless this is very stringently monitored and controlled.

Hello julie, I picked up your suggestion, I have now set up with a single project-layer.
I wanted to ask you one last thing: with the method you suggested, is it possible to see not the worked hours but, for each resource, the ratio of the worked hours on a project and the total working time actually carried out in a month? For example, resource 1 works 75h for project 1 and 25h for project 2, the rest of the month is non working time: I would like show that in the month resource 1 spends 0.75 of working time for project 1 and 0.25 for project 2. I could do it in excel, I wonder if it is possible to do it automatically in Project

Work is shown in hours but you can add the Percent Allocation field to the detailed (right hand side) of the view. That will show you the percent of maximum units the resource is working. It will show in percentages.

Hello JulieS, it seems to work, but not completely: infact, the method indicates the hours and the percent allocation of each task that composes a subproject, it also shows the total hours of the subproject, but not the overall percent allocation of the subproject. It 's strange!!!