how to convert the last 3 characters on a string into an int

const char* get_substring(const char* str) {
  char* res = new char[4];
  strncpy(res, str + strlen(str) - 3, 3);
  return res;
}

int convert_substring_to_int(const char* str) {
  int result = 0;
  for (int i = 0; i < 3; i++) {
    result = result * 10 + (str[i] - '0');
  }
  return result;
}

int process(const char* str) {
  return convert_substring_to_int(get_substring(str));
}