input: Write a python function to find the maximum of three numbers.
  assert max_of_three(3, 4, 5) == 5
  assert max_of_three(10, 5, 7) == 10
  assert max_of_three(1, 2, 3) == 3
output: def max_of_three(a, b, c):
      return max(a, b, c)