input: Write a python function to encode a string by shifting each character by 3 positions.
  assert encodeString("Hello") == "Khoor"
  assert encodeString("abc") == "def"
  assert encodeString("xyz") == "abc"
output: def encodeString(s):
      encoded_chars = []
      for char in s:
          encoded_chars.append(chr((ord(char) - ord('a') + 3) % 26 + ord('a')))
      return ''.join(encoded_chars)