input: Write a python function to find the maximum of three numbers.
  assert max_of_three(4, 7, 6) == 7
  assert max_of_three(10, 5, 3) == 10
  assert max_of_three(1, 2, 3) == 3
output: def max_of_three(a, b, c):
      return max(a, b, c)