Prolog - How to generate a number with certain prerequisites?

So I want to create a simple number generator that generates a number between 1 and 9, but it is not allowed to be part of three lists provided (lists of numbers). An example:

findnumber(Number, [1,2,3], [3,4,5], [6,7,8]).
Number = 9.

or:

findnumber(Number, [1,2], [3,4], [5,6]).
Number = 7;
Number = 8;
Number = 9.

How would I go about this, I tried this:

findnumber(Number, List1, List2, List3) :-
  random_between(1, 9, Number),
  not(member(Number, List1)),
  not(member(Number, List2)),
  not(member(Number, List3)).

I thought this would work but apparently not, I think it is because the Number is generated beforehand so it can't really find the prerequisites. It merely checks if they aren't members and if they are, then the predicate fails.

Hopefully someone can help me out. Thanks in advance.