input: Write a function to find the median of three specific numbers.
  assert median_numbers(25,55,65)==55.0
  assert median_numbers(20,10,30)==20.0
  assert median_numbers(15,45,75)==45.0
  
output: def median_numbers(a, b, c):
      if (a >= b and a <= c) or (a <= b and a >= c):
          return float(a)
      elif (b >= a and b <= c) or (b <= a and b >= c):
          return float(b)
      else:
          return float(c)