"Pleasant" and "pleasing" are very similar and are often used interchangeable, but there is a difference. "Pleasant" should be used to describe something that has naturally appealing qualities. "Pleasing" should be used to describe something that was deliberately meant to please a person. - She has a very pleasant smile. - Her behavior was very pleasing. Again, though, people in the USA use these two words to mean the same thing most of the time. "Pleased" is sort of the opposite of the other two (In the sense that "catch" and "throw" are opposites) - He was pleased by her smile. - He was pleased by her behavior.