input: Write a function to find the maximum of three numbers. 
  
  def max_of_three(a, b, c):
      if a > b and a > c:
          return a
      elif b > a and b > c:
          return b
      else:
          return c
  
  assert max_of_three(3, 4, 5) == 5
  assert max_of_three(10, 20, 30) == 30
  assert max_of_three(-1, -2, -3) == -1
  assert max_of_three(4, 4, 4) == 4
  assert max_of_three(15, 10, 20) == 20
output: def max_of_three(a, b, c):
      if a > b and a > c:
          return a
      elif b > a and b > c:
          return b
      else:
          return c