036 Three Chars Distinct

given a string s, find out all the substring of this string with length equals to 3 that all of its three chars are distinct.
get the number of substring that meets the requirements. return the value.

this problem is pretty straight forward too.
but I’m thinking there might be better solutions than this brute force.

public int threeCharsDistinct(String s) {
  if (s == null || s.length() < 3) return 0;

  int i = 0;
  int res = 0;
  while (i <= s.length() - 3) {
    if (checkDistinct(s.substring(i, i + 3)) != 0) { //equals 0 means we have duplicates
      res++;
    }
    i++;
  }
  return res;
  
}

private int checkDistinct(String s) { //the length of it must be 3
  int res = s.charAt(0);
  int i = 1;
  while (i < s.length()) {
    res = s.charAt(i)^res;
    i++;
  }
  return res;
}

the above code is wrong because:
although a ^ b != 0 and b ^ b == 0
but a ^ b ^ b != 0 and b ^ b ^ a != 0

public int threeCharsDistinct(String s) {
  if (s == null || s.length() < 3) return 0;

  int i = 0;
  int res = 0;
  while (i <= s.length() - 3) {
    if (checkDistinct(s.substring(i, i + 3)) != 0) { //equals 0 means we have duplicates
      res++;
    }
    i++;
  }
  return res;
  
}

private int checkDistinct(String s) { //the length of it must be 3
  HashSet<Character> set = new HashSet<>();
  int res = 1;
  int i = 0;
  while (i < s.length()) {
    if (set.contains(s.charAt(i))) {
      return 0;
    } else {
      set.add(s.charAt(i))
    }
    i++;
  }
  return res;
}