A special case length < 3 seems like a bug. A string aa is a palindrome, and one could successfully argue that a single-letter string is also a palindrome.

A boolean isPalindrome is redundant, and forces the code to test the effectively same condition twice. An early return is perfectly fine here.

A traditional idiomatic (and arguably more performant) way to iterate forward is *first++. Similarly, an idiom for iterating backwards is *--last:

      while (first < last) {
          if (*first++ != *--last) {
              return false;
          }
      }
      return true;

The naming is very inconsistent. I don't see the reason to spell palindrome in 3 different ways. A capitalization in iSPalendrome is also strange.

A special case length < 3 seems like a bug. A string aa is a palindrome, and one could successfully argue that a single-letter string is also a palindrome.

A boolean isPalindrome is redundant, and forces the code to test the effectively same condition twice. An early return is perfectly fine here.

A traditional idiomatic (and arguably more performant) way to iterate forward is *first++. Similarly, an idiom for iterating backwards is *--last (that also avoids subtracting 1 beforehand):

      last = possiblePalindrome.end();
      while (first < last) {
          if (*first++ != *--last) {
              return false;
          }
      }
      return true;

The naming is very inconsistent. I don't see the reason to spell palindrome in 3 different ways. A capitalization in iSPalendrome is also strange.