Solve issue #1876

[Smithech]

Solve “Substrings of Size Three with Distinct Characters” (#2666)

Add def good_substrings(s: str) -> int

• The good_substrings function iterates through a string and counts all substrings of size 3 with distinct characters.
• Comments and a docstring are included to help beginners understand how it works.

Example: count_distinct_substrings("abcabc") returns 4.
Explanation: The good substrings are 'abc', 'bca', 'cab', 'abc'.