Write all possible 3 digit number whose sum of digit is 4

Answer 1

Just consider the possible sums, then arrange the digits:
4+0+0 = 4 ---> 400
3+1+0 = 4 ---> 310 301 130 103
2+2+0 = 4 ---> 220 202
1+1+2 = 4 ---> 112 121 211