From the problem we can gather that $ 20<C<30 $. Because $ C $ is also a multiple of 7, that limits the possibilities for number $ C $ down to 21 and 28. The last clue left states that number $ C $ is also NOT a multiple of 3. Out of the two posibilities left, 21 is the only value that is also a multiple of 3, thus eliminating it from being number $ C $. Thus, the number $ C $ can only be $ \boxed{28} $.