Let the balls in the bag be represented by the 3tuple (r,w,b);
where r, w, and b are the number of red, white, and blue balls
mod 3. The starting position is (0,1,2), the required ending
position is 0,0,0), and for the procedure 
(n1)mod 3 = (n+2)mod 3. (0,1,2) >
(2,0,1) >
(1,2,0) >
(0,1,2) >
repeat
Can never reach position (0,0,0).
QED
