Hi I think u need to create an expression in such a way that nothing is selected change ur expression to select 3 with rank funciton.
for example if u have Year, Quarter, Month Date.
Expression 1: select the top 3 values when year, quarter , date and month writing IF conditions. with rank<=3 expression
Second. if they select anything from calendar try to display all the values with regualt expression.
If still u need some help post with a sample QVW file with what selections u want to display top 3 by default .