Please solve the programming task below using a self-contained code snippet in a markdown code block.

A string is good if there are no repeated characters.
Given a string s​​​​​, return the number of good substrings of length three in s​​​​​​.
Note that if there are multiple occurrences of the same substring, every occurrence should be counted.
A substring is a contiguous sequence of characters in a string.
 
Please complete the following python code precisely:
```python
class Solution:
    def countGoodSubstrings(self, s: str) -> int:
```