old_contents: #!/usr/bin/env python
  
  
  def inputNumber():
      counter = 0
      numbers = list()
      while True:
          num = input('Enter number:')
          if isinstance(num, int):
              numbers.append(num)
              counter += 1
          else:
              print ('Not number: %s' % num)
              continue
          if counter == 3:
              break
      return numbers
  
  
  def max(list):
      max = sorted(list, reverse=True)
      return max[0]
  
  
  def main():
      a = inputNumber()
      b = max(a)
      print('Max of 3 digits %s is: %i' % (a, b))
  
  if __name__ == '__main__':
      main()
  
new_contents: #!/usr/bin/env python3
  
  
  def inputNumber():
      counter = 0
      numbers = list()
      while True:
          num = int(input('Enter number:'))
          if isinstance(num, int):
              numbers.append(num)
              counter += 1
          else:
              print ('Not number: %s' % num)
              continue
          if counter == 3:
              break
      return numbers
  
  
  def max(list):
      max = sorted(list, reverse=True)
      return max[0]
  
  
  def main():
      a = inputNumber()
      b = max(a)
      print('Max of 3 digits %s is: %i' % (a, b))
  
  if __name__ == '__main__':
      main()
  
subject: Migrate to python 3. Input in python 3 works like input_raw in python 2, so to convert data during input i have to explicitly convert to ints, with int() function