Given a string s, return the count of unique palindromic subsequences of length $3$ that exist within s.

Even if a particular subsequence can be formed in multiple ways, it is counted only once.

A palindrome is a string that reads the same forwards and backwards. A subsequence is derived from the original string by deleting zero or more characters without altering the relative order of the remaining characters.

Constraints:

• $3 \leq$ s.length $\leq 10^5$

• s consists of only lowercase English letters.

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