Not sure I fully understand.
What expressions / calculations are you using in your three expressions? Wouldn't a single expression with drill down dimension group do what you want? In other words, do you need to add a different expression when drilling down?
Or do you want to use mutual exclusive expressions, then maybe add to the conditionals something like
Line: GetSelectedCount(Group)=1 AND GetSelectedCount(Line)=0 AND GetSelectedCount(Network)=0
Network: GetSelectedCount(Line)=1 AND GetSelectedCount(Network)=0
Profit Center: GetSelectedCount(Network)=1
Or check the drill down group dimension using GetCurrentField( GroupName) in the expression conditional.
Hope this helps,