Three letters are written to three different persons and addresses on the three envelopes are also written. Without looking at the addresses, the letters are kept in these envelopes the probability that all the letters are not placed into their right envelopes is:

5/6

All are not in right envelopes = 1 - P (all in right envelopes) 

$=1-\frac{1}{3!}=\frac{5}{6}$