input: Write a python function to get the first three characters of a string.
  assert get_first_three_chars("Python") == "Pyt"
  assert get_first_three_chars("Hello World!") == "Hel"
  assert get_first_three_chars("Hi") == "Hi"
  assert get_first_three_chars("") == ""
output: def get_first_three_chars(s):
      return s[:3]