On page 154 of "Elementary Number Theory" by Underwood Dudley, problem 8 states
If $n=x^2+y^2+z^2+w^2$, show that by suitable ordering and choices of sign we can get $x+y+z$ to be a multiple of $3$.
What does "get $x+y+z$ to be a multiple of $3$." mean in that sentence ? Do I have to show that, in any sum of four squares, three of the integers add up to a multiple of $3$ ?