Trigrams are a special case of the n-gram, where n is 3. One trigram is a continious sequence of 3 chars in phrase. [Wikipedia](https://en.wikipedia.org/wiki/Trigram)
 - return all trigrams for the given phrase 
 - replace spaces with \_ 
 - return an empty string for phrases shorter than 3
Example:
trigrams('the quick red') ==  the he\_ e\_q \_qu qui uic ick ck\_ k\_r \_re red
def trigrams(phrase):
    phrase = phrase.replace(' ', '_')
    return ' '.join(''.join(xs) for xs in zip(phrase, phrase[1:], phrase[2:]))
def trigrams(phrase):
    phrase = phrase.replace(' ', '_')
    return ' '.join(''.join(xs) for xs in zip(phrase, phrase[1:], phrase[2:]))