For each positive integer n, let S(n) denote the sum of the digits of n. How many three-digit n's are there such that

For each positive integer n, let S(n) denote the sum of the digits of n. How many three-digit n's are there such that  

$n+S(n)+S(S(n))\equiv 0 \pmod{9}?$

Thanks for helping! Also- please explain your answer so I can better understand it!