input: Write a python function to find the top 3 most frequent words in a given string.
  assert top_3_words('This is a sample string sample sample') == ['sample', 'This', 'is']
  assert top_3_words('apple apple orange orange banana') == ['apple', 'orange', 'banana']
  assert top_3_words('one two two three three three') == ['three', 'two', 'one']
output: def top_3_words(s):
      from collections import Counter
      words = s.split()
      count = Counter(words)
      most_common = count.most_common(3)
      return [word for word, freq in most_common]