if all 3 levels are interdependent I don't believe it will work as desired...
Grouping is hierarchical, so group 1 will tend to dominate. As described you will get the top event, and you could then see the top regions associated with that event and the top cities for the top region.
if you want to see the top events, then the top regions irrespective of the event, then I would think that you are looking at a subreport, otherwise your grouping will always be tied to the top events
HTH