In my 25M word corpus, "this is fun" occurs 23 times. There are only 94,000 trigrams that occur more frequently.
Therefore, you should be pessimistic, and consider the password "this is fun" less safe than passwords in the shape [a-zA-Z]{3}, like "tsP", of which there are 140608.
Assume attackers know the algorithm (e.g. three common words, one 7-letter word in l33tspeak, a 6-letter string of random ascii characters) but not the parameters.
Therefore, you should be pessimistic, and consider the password "this is fun" less safe than passwords in the shape [a-zA-Z]{3}, like "tsP", of which there are 140608.
Assume attackers know the algorithm (e.g. three common words, one 7-letter word in l33tspeak, a 6-letter string of random ascii characters) but not the parameters.