input: Write a python function to multiply each element in a list by 3.
  assert multiply_by_three([1, 2, 3, 4]) == [3, 6, 9, 12]
  assert multiply_by_three([5, 10, 15]) == [15, 30, 45]
  assert multiply_by_three([0, -1, -2]) == [0, -3, -6]
output: def multiply_by_three(lst):
      return [x * 3 for x in lst]