input: Write a python function to find the maximum of three numbers.
  assert maximum_of_three(10, 20, 30) == 30
  assert maximum_of_three(5, 15, 10) == 15
  assert maximum_of_three(-1, -2, -3) == -1
output: def maximum_of_three(a, b, c):
      return max(a, b, c)