Show lowest granular prompt when multiple prompts selected

Hi All,

I need your help here, I have created a cross tab report which has 3 level of User prompts, as U1,U2 and U3, all are InList, now what I need is when I select values in U1 & U2 prompts i want only the U2 prompt values should be displayed on the report
Similarly When U2 and U3 selected I want to see U3 alone in Report, and when U1 & U3 then i wnat to see only U3 values.
only take the lowest granularity

Note U1,U2&U3 are in Hirearchy, U3 reporting to U2 which inturn reports to U1

I need your help on this .Thanks in Advance