They are added to people's names. -san is like mr., ms. or mrs. -chan is used when you're casual with the person and it's better to use it on someone who's younger and is a girl, or a kid. It also sounds cute. Haha. -kun is used for boys, teens or kids.

use "san" for formal use use "chan" for small girl or teenager that you already know well use "kun" for little boy or your subordinate

use "san" for formal use use "chan" for small girl or teenager that you already know well use "kun" for little boy or your subordinate