input: Write a python function to find the maximum of three numbers.
  assert find_max(7, 5, 9) == 9
  assert find_max(3, 4, 2) == 4
  assert find_max(10, 10, 5) == 10
output: def find_max(a, b, c):
      if a >= b and a >= c:
          return a
      elif b >= a and b >= c:
          return b
      else:
          return c