input: Write a python function to find the three most frequent words in a string and return them in a list.
  assert most_frequent_words('It is very important to be pythonic when writing python code.') == ['to', 'be', 'code.']
  assert most_frequent_words('hello hello world world world') == ['world', 'hello']
  assert most_frequent_words('a b c d e f g h i j a a b b c c') == ['a', 'b', 'c']
output: def most_frequent_words(text):
      from collections import Counter
      
      words = text.split()
      count = Counter(words)
      most_common = count.most_common(3)
      
      return [word for word, freq in most_common]