This response is way late, but there is a way to do this. What I think you're getting at is something along the lines of overloading (having duplicate functions with different parameters such as f(a,b) and f(a,b,c)). I don't think Nspire basic can do this, but you could give your input as a list (such as f({a, b, [c]}) where the list is essentially just one variable (it can be filled with booleans, strings, numbers, other lists, etc). Then you could have conditions at the beginning of your code to respond to the length of the list (using dim()).
Define f(lst)=
Prgm
If dim(lst)=2 Then: Disp "Two parameters"
ElseIf dim(lst)=3 Then: Disp "Three parameters"
Else: Disp "Input error"
Endif
EndPrgm
You would want to replace the display functions with whatever you want your program to do with the input. You probably already figured out a solution, so maybe this will help someone else. Cheers