"this is fun" has structure and I suspect is an easy password to guess from the pool of all three word passwords. Just like using a dictionary is better than brute force, trying common words that usually go together when guessing three word passwords is much better than trying all three word passwords. If Google were to write a "word" password cracker using their data trove, I suspect "this is fun" would go down early. Likewise putting spaces between words would be to the Google cracker like adding a number on the end of a dictionary word is to a standard cracker.