input: Write a function to find all palindromes of length at least 3 in a given string.
  assert find_palindromes("racecar") == ["racecar", "aceca", "cec"]
  assert find_palindromes("banana") == ["ana", "anana", "nan"]
  assert find_palindromes("abc") == []
output: def find_palindromes(s):
      def is_palindrome(sub):
          return sub == sub[::-1]
  
      n = len(s)
      palindromes = []
      for i in range(n):
          for j in range(i+3, n+1):
              substr = s[i:j]
              if is_palindrome(substr):
                  palindromes.append(substr)
      return palindromes